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\begin{document}

\title{Large-scale siting of sand dams: a participatory approach and
application in Angolan drylands}



\author[1]{Luigi Piemontese}%
\author[2]{Natalia Limones}%
\author[1]{Giulio Castelli}%
\author[3]{Alice Grazio}%
\author[1]{Elena Bresci}%
\affil[1]{Università degli Studi di Firenze}%
\affil[2]{Universidad de Sevilla}%
\affil[3]{COSPE Onlus}%


\vspace{-1em}



  
  \date{June 13, 2024}


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\selectlanguage{english}
\begin{abstract}
Sand dams are simple and effective structures built across ephemeral
riverbeds in arid/semi-arid regions to harvest water within sand pores
and increase water availability and quality for rural communities. The
complex morphological, hydrological, social and economic conditions that
make sand dams a beneficial tool for water resilience are largely
influenced by the siting phase. Proper location of a sand dam can reduce
community's travel time to water points, reduce water conflicts and
increase food security through expansion of irrigated agriculture. On
the other hand, a misplacement of sand dams can, at worst, increase
disparities in water access and increase local conflicts. To approach a
viable siting of sand dams, most projects are developed and delivered
with the community through a bottom-up approach. However, in case of
large-scale project, remote sensing and biophysical analysis are the
dominant approach, leaving the socio-economic component at the margins
of the siting strategy and eventually affecting the benefits to local
communities. In this paper, we propose a large-scale participatory
methodology to sand dams siting, which draws on mixed-methods connecting
the conventional top-down biophysical analysis with bottom-up
participatory research. We first describe the generic approach developed
for sand dams siting in Namibe, a semi-arid region of South-west of
Angola, then we draw on our case to propose a generic approach to
large-scale participatory siting beyond Namibe.%
\end{abstract}%



\sloppy




\textbf{Introduction}

Water scarcity is one of the major threats to human wellbeing and a
central challenge to achieving the milestones of the sustainable
development goals (Gain et al., 2016) and ecological restoration
(Fischer et al., 2021). Drylands are particularly vulnerable areas
characterized by extreme water scarcity, variable rainfall and sparse
vegetation, covering almost 40\% of global land surface (Safriel et al.,
2005). Because of the unstable and uncertain natural resources, drylands
are often home to marginalised populations (Sietz et al., 2011), who
have adapted to resources scarcity and uncertainty, thus developing
resilience to environmental changes (Ifejika Speranza, 2010). However,
climate change is expected to expand drylands by changing precipitation
patterns and intensity (Feng and Fu, 2013), with serious consequences of
increasing populations out-migrations and armed conflicts (Neumann et
al., 2015; Sterzel et al., 2014). For example, Southern African drylands
are expected to see a decrease in precipitation, runoff and soil
moisture, which will likely affect the resilience of millions of
agro-pastoralists (Piemontese et al., 2019).

Retaining water in the landscape has a critical role for increasing
resilience of drylands to climate change ``by avoiding major regime
shifts away from stable environmental conditions, and in safeguarding
life-support systems for human wellbeing'' (Rockstr\selectlanguage{ngerman}öm et al., 2014).
Harvesting rainwater through small-scale and distributed interventions
can be a crucial strategy to increase water retention in the landscape,
increasing pasture and water availability for agro-pastoralists
(Descheemaeker et al., 2010; Strohmeier et al., 2021).

Within the broad spectrum of water harvesting technologies, sand dams
represent a particularly interesting and promising technology to harvest
water in dry rivers. Sand dams are barriers built along ephemeral
rivers, which trap coarse sediments (mostly sand), creating a small
reservoir filled in sand (Ritchie et al., 2021). The sand trapped behind
the dam serves as a water storage -- up to 40\% of the sand volume are
pores that can be filled in water (Lasage et al., 2015). Sand dams have
demonstrated successful in providing additional water points along
ephemeral rivers and increasing communities wellbeing (Teel, 2019). In
Kenya, where the majority of the research on sand dam is focused, sand
dams have decreased the water collection time from more than 3 hours per
day to as little as 15 minutes (Teel, 2019). Other benefits of sand dams
include increased water use of about 3 times, increased irrigation
water, thus increased food security (Ritchie et al., 2021; Villani et
al., 2018).

Replicating and transferring water harvesting (WH) solutions like sand
dams to other areas of the world could boost land and water restoration
targets and contribute to strengthening social-ecological resilience of
drylands (Ammar et al., 2016; Grum et al., 2016; Piemontese et al.,
2020). However, especially in areas with no previous experience,
planning the most appropriate location for implementation is a key step
to ensure the usefulness and effectiveness of the technology. In fact,
some 80\% of the WH implementations fail because of wrong siting for
both failed water retention and scarce involvement of local communities,
who are the ones using and benefiting from the technologies (Ngugi et
al., 2020).

This is the case of Angola drylands, where sand dams have not been
documented in the scientific literature (Ritchie et al., 2021), but the
successful experience of places with similar hydro-climatic conditions
in Kenya and in the near Namibia (Hartley, 1997), makes it an appealing
solution to water scarcity. However, especially in a new context,
exploring the feasibility of such technology requires a deep
understanding and careful inclusion local communities perceptions and
knowledge, not only for selecting a suitable site, but also for ensuring
a long-lasting beneficial use (Ziadat et al., 2012).

Conventionally, the siting of sand dams for large areas (regional to
national) is approached as a top-down, data-driven analysis to highlight
the areas, or river sections, with adequate biophysical conditions for
sand dams construction (Forzieri et al., 2008). These large-scale
biophysical studies have the advantage to effectively reduce the time
and resources for a more detailed field inspection in large areas, but
they require high resolution spatial data and often lack socio-economic
information on technology viability and use. On the other hand, higher
details of suitability analysis can be reached with local participatory
approaches, which include a more thorough socio-economic enquire and
community participation, but are limited in spatial scale (Ertsen et
al., 2005). Although these two approaches seem polarized, and in fact
they have been applied in silo, an integration is still possible and
desirable to produce insightful large-scale assessments.

Here, we present a mixed-method approach to large scale siting
considering both the biophysical requirements of a sand dam site and the
needs, constraints and preferences of local communities. We apply the
approach to Namibe, a semi-arid Province in Southwest Angola, by using a
participative enquiry to assess the water security problems of the
communities, analysed to incorporate the socio-economic conditions for
the best siting of sand dams. The twofold contribution of this work
tackle both the generic research gap on large-scale participatory
planning of water harvesting technologies and contributes to expanding
the geographical understanding of sand dams in a data-scarce and
understudied region as the Angolan drylands.\textbf{{}}

\textbf{Data and Methods}

\textbf{2.1 Methodological framework}

To identify the sites with highest potential for sand dams in the
province of Namibe, we use a mixed-methods approach building on
quantitative and qualitative methods applied to ten communities in the
two \emph{municípios} of Virei and Bibala. This approach was
specifically developed to meet two major challenges in the siting of
water harvesting in drylands. First, drylands, especially in Sub-Saharan
Africa, are often areas with poor data availability and difficult access
to information (Prinz et al., 1998). Lack of high resolution spatial
biophysical data is a major limitation to estimating a complex water
harvesting technology as sand dams. A suitable river section for sand
dams construction must have specific characteristics such as presence of
high rocky riverbanks, adequate size of the stream, shallow bedrock,
coarse and little clayish sediments and low salinity (Beswetherick et
al., 2018; Ngugi et al., 2020). Although some of these data might be
available, they usually come from global-scale datasets, rarely tested
in Sub-Saharan Africa and at a coarse resolution, which could compromise
the reliability of the analysis. The second challenge is represented by
the complete lack of socio-economic data. Although with some
uncertainty, the literature on best-siting of water harvesting
structures suggests the use of socio-economic parameters as distance to
communities, rivers, roads, land tenure, population density, workforce
availability or prior community experience with similar infrastructures.
However, these kinds of data are rarely available and particularly
lacking in Angola, where most of scientific and technical work is
outdated and definitely neglected remote areas such as rural Namibe
(Amado et al., 2020).

\textbf{2.2 Knowledge integration}

To overcome these problems, instead of aiming for the exact location of
such a complex technology, we first performed a classic top-down siting
with few essential biophysical criteria to narrow down the suitable
river sections with the basic biophysical requirements for sand dams
construction and then we used this preliminary points to guide the local
participatory research, which completes the overall social-ecological
suitability. In this, we diverge from conventional top-down large-scale
siting which seldom, if ever, include a local participatory process to
``validate'' the top-down methodology and do not fully integrate local
perception or indigenous knowledge (Ziadat et al., 2012). With our
methodology (see Figure 1), we build on qualitative GIS to map
communities' needs, constraints and use their knowledge of local
hydrology to integrate out top-down biophysical analysis with missing
local social and hydrological information which constitute the ``useful
knowledge'' (Robinson et al., 2016), which is essential to select the
sites with the best overall suitability for sand dams construction
(Forrester et al., 2015). Particularly relevant for the knowledge
integration phase was the field inspection. After the focus group
discussions, we showed the map with the biophysically suitable points to
a group of key informants (usually the \emph{soba,} the local
administrator, and/or other distinguished community members), asking
whether the points were located in \emph{mulola} (i.e. local term for
ephemeral rivers) with high rocky river banks and if they had good sand.
Also, we asked which are the main \emph{mulola} with \emph{cacimbas}
(i.e. scoop holes), which is a good indicator of potential for sand dams
(Beswetherick et al., 2018). Apart from refining the selection of
biophysical points in this way, we also asked directly to the community
whether they had ideas or suggestions for selecting a site. Although
this approach is subjected to individuals' own interest and convenience
(for example recommending a river section near their settlements or
\emph{kimbos}), it was a useful exercise to allow the community for a
more active engagement, build trust and reciprocity and let the
community show their priorities and preferences without constraints.

\textbf{2.3. Study area}

Case study selection

The province of Namibe is located Southwest of Angola, bordering
Namibia, and hosts about 1,200,000 people. The main activity of rural
people is animal farming, mostly goats and cows, with official
statistics estimating the cattle size to 500.500 heads (Angola, 2013).
However, the remote location of the largest cattle keepers and their
semi-nomadic nature linked to mobile transhumance practices make it
difficult to reliably quantify. In this area, cows are considered much
more than a subsistence mean or a financial and capital, but they
represent a ``spiritual capital'', connecting people to their ancestors
(Bagnol and Verhoesel, 2009). For this reason, taking care of the
animals is the main activity of the Namibian pastoral communities, who
also keep animals in close proximity to their families. Farms
(\emph{sambos}) are usually embedded within the clans' settlements
(\emph{kimbos}), surrounded by a larger area where family houses and
other facilities are spread. Within the \emph{Mukubal}, the largest
cattle keepers populating mostly the Virei area, the lives of people and
animals are so intertwined and co-dependent that the settlements are
even called just \emph{sambos}.

The scarce and increasingly unpredictable rainfall make Namibe prone to
long droughts and extensive water scarcity, which are already severely
affecting the livelihood of local communities (Limones et al., 2020).
Local communities have traditionally been practicing transhumance to
cope with water scarcity and land degradation, by seasonally reaching
locations with higher availability of resources for their livestock
(Carvalho, 1974). However, climate change is worsening the
hydro-climatic conditions of the areas, forcing local communities to
change and prolong their transhumance routes with increasing risk of
failure (Herrero et al., 2016).

The ten communities involved in this study are located in three
different sub-municipal divisions (the \emph{comunas} of Cainde,
Capangombe and Lola), spanning across different socio-economic and
agro-climatic areas in a north-south gradient, from the southern
semi-arid livestock-based communities of Cainde, to the agro-pastoralist
communities of sub-humid Lola. Capturing the social-ecological gradient
of the ten communities is fundamental to understand the different
socio-economic conditions for siting sand dams along the most important
transhumance route of the Province. The three \emph{comunas} are in fact
connected by the virtual cattle road that brings pastoralist from drier
areas to greener and more humid areas. The light grey area in Figure 2
is the official administrative territory of the communities, which have
their own local \emph{soba}, but it often extends further beyond those
boundaries to include further pasture and water points, which
transhumant pastoralists are extremely dependent on (Mufinda et al.,
2015).{}

\textbf{2.4 Biophysical suitability: multivariate analysis}

The first step of our approach is the selection of biophysically
suitable location in the province of study. This step is necessary to
identify a first set of location with the necessary biophysical
condition for the large area of the ten communities. Moreover, being
mobile pastoralists, the communities could make good use of water points
located even tens of kilometres far from the core of their villages.
Selecting suitable location in such a large area would be impossible
with a pure participatory work. We then use Multi-criteria Decision
Analysis (MCDA) to select a first set of points for the participatory
phase. ~MCDA uses a set of parameters to discriminate between suitable
and unsuitable conditions leading to a certain result. It is a
consolidated approach in siting suitability analysis of water harvesting
structures (Al-Adamat et al., 2010; Forzieri et al., 2008; Grum et al.,
2016). In a recent review of siting methods, Ammar et al. (2016) reports
MCDA as one of the most reliable approaches, especially when combined
with spatial analysis tools, since it allows to analyze several spatial
indicators for large areas in a quick and inexpensive way. {}

Given the uncertainty in selecting the most adequate parameters for the
biophysical suitability of sand dams (Ngugi et al., 2020) and the
mentioned lack of reliable and high resolution data, we adopted a
conservative approach using a simple set of parameters representing the
minimum requirement of measurable variables for the construction of sand
dams, including the slope, the salinity and the stream order. The
purpose of the analysis is therefore to narrow down the potential number
of water stream sections to be inspected during the second phase in the
field. {}

The analysis included the delineation of the stream and the calculation
of the stream order (or Strahler order), and the subsequent selection of
the points along the stream with appropriate conditions of stream order,
slope and salinity. Hereinafter we report the analytical steps with some
explanatory figures.{}

To select the final set of water stream sections we have first produced
the hydrographic network (i.e. water streams) starting from the Digital
Elevation Model (DEM), using the GIS functions slope, aspect and
exposition applied at pixel level from the DEM grid. From the slope and
exposition layers, we then calculated the flow accumulation, which is
the number of upstream cells draining into each cell of the grid. From
the flow accumulation we then reconstructed the stream network by
selecting the threshold of 1000 cells from the flow accumulation layer.
After reconstructing the river layer, we broke the stream lines into
sections 30m apart, technically by converting the stream lines into
points. We yield a total of 1813428 points, which represent all the
stream sections in Namibe, then we narrowed down the selection of
suitable point by excluding the points located in areas with slope
higher than 2 degrees, stream order higher than 2 and soil salinity
higher than 40Hz. The stream order is a proxy for stream dimensions,
since sand dams are most suited for river sections below 25m
(Beswetherick et al., 2018). Finally, soil electrical conductivity below
40Hz is a common acceptable salinity value for drinking use (following
the FAO guidelines for maximum electrical conductivity values:
\url{http://www.fao.org/3/x5871e/x5871e04.htm}).

2.4.1 Field calibration of biophysical criteria

The field inspection for the model calibration involved the selection of
3 random analysis points located in an easily accessible location. The
purpose of the field inspection was to check whether the dimensions of
the selected stream order (between 1 and 2) actually correspond to
stream dimensions which are feasible for the construction of small sand
dams. The characteristics checked in the field where thus the slope and
the cross section between 10 to 20 meters. During the participatory
research with the communities, we had further proof that the
\emph{mulola} with section range 10-20 m are the most suitable because
smaller \emph{mulola} (with section below 10 m) were not used by locals
since they are too small to accumulate water in the sand for a
reasonable time to make the digging work worth it.

\textbf{2.5 Qualitative data collection and analysis}

To understand the context-specific socio-economic criteria for the
best-siting of sand dams, we have conducted qualitative research with
the 10 communities in Namibe, collecting information on water
availability, water use and needs based on participants perception. We
performed three focus group discussions in each community, including a
representation of participants of both sexes and all ages (Figure 3).
The groups were previously engaged and initiated in participatory work
by the NGO leading the cooperation project TransAgua, to build a certain
degree of trust and collaboration throughout the participatory
activities. Focus group discussions were held in August 2021 and
facilitated by the first author and local facilitators of the NGO, who
also helped with the logistics and translation from Portuguese to the
local language. The topics of the focus groups concerned water
availability (i.e. length and intensity of the rainy season and stream
water availability and timing), water use and needs of transhumant
pastoralism (i.e. animal water consumption and dynamics related to water
needs and availability during transhumance) and domestic water (i.e.
quantity, quality and location of local water sources and water and
domestic water use).

During the group discussions, we also asked about the familiarity of the
communities with water harvesting structures and the possible presence
of small dams in the area. The presence of sand dams in the area has not
been reported in scientific or grey literature, but we were expecting
the presence of colonial water structures to provide water to the
``modern'' large-scale livestock ranching in Namibe during the late
colonial time (Carvalho, 1974).

The focus group discussions were facilitated using the semi-structured
interview guide available in supplementary material. Interviews were
reported using edited transcription to avoid repetitions and to increase
readability and clarity, particularly useful when translating answers
between English, Portuguese and the different local languages. The lead
author then coded the transcripts using a mix of deductive and inductive
coding. The scope of the qualitative analysis was to identify the
socio-economic criteria to be considered for the siting of sand dams. To
come up with a list of criteria that would account for the communities
water needs and priorities, we first identified the main water security
themes of the communities. The water security themes were first
deductively coded guided by the water security framework proposed by
Loring et al. (2013), which is broad enough to include other potential
emerging themes specific to the local context (Penn et al., 2017). The
framework categorises water security based on availability,
accessibility, utility, and stability of water resources. Water
availability is the water considered to be extractable in and around the
community; accessibility concerns the degree of access the water points
(e.g. having a well in the village makes water more accessible than
having to dig a hole in a sandy river far away from the village);~
utility refers to the level of safety in water accessibility and use
(e.g. polluted or salty water); finally, stability indicates reliability
of water resources over time, including the presence of water shortage
periods or natural disasters (e.g. droughts and floods).

Starting with coding for these themes we then inductively added other
potential water security categories emerging from the interviews. The
lead author coded the focus group discussions and reviewed sections of
the coded interviews with the local facilitator to make sure to properly
interpret the transcripts and to discuss preliminary themes.

After the identification of the themes related to water security
problems, we gave a 5-grade score to each problems based on their
severity from none, to low, medium, high and very high according to
participants' perception. Finally, based on the set of problems
identified with the communities and their severity score, we associate a
socio-economic criteria for the best-siting of sand dams addressing the
identified problems.

During the whole participatory process, we triangulated the results from
the focus groups with notes from participant observations (Hertel,
1974), field visits with key informants from the communities as well as
conversations with other experts involved in development projects with
pastoral communities based in Namibe.

\textbf{Results}

\textbf{3.1 Biophysical suitability}

After applying the filters of the multivariate analysis to the total
river sections, we obtained 2106 suitable points in the whole Namibe
province (Figure 4, left). This number goes down to 121 points if we
only consider the ones located within the territory of the 10
communities (Figure 4, right). This is the final set of points used as a
guidance in the participatory phase. It is important to notice that,
given the small number of conditions included in the analysis, many of
these points might not present all of the specific site conditions
required for a dam construction; for example strong rocky riverbanks.
Moreover, many of these points are likely to be located in remote areas
that would not be useful in providing water to local communities, such
as in the large Namibe desert covering the southern part of the
province, bordering Namibia. For this reason, integrating this analysis
with local communities knowledge and socio-economic constrains and
requirements is a fundamental integration to the final siting of sand
dams in this case.

\textbf{3.2 Socio-economic conditions for best siting of sand dams in
Namibe}

The main themes related to water security emerged from the qualitative
analysis of focus group discussions are the following:

1.~~~~ Water scarcity: The overall water quantity available for the
community is not enough for the different uses.

2.~~~~ Few water points: The community has access to a limited number of
water points, including larger \emph{mulola} for \emph{cacimbas}, wells
and other non-conventional sources.

3.~~~~ Difficult access: The water sources are hard to access. For
example the community have to dig several or very deep \emph{cacimbas}
to collect water.

4.~~~~ Unequal access: Families within the community are located far
from the water points and have to walk for considerably longer time
compared to the families living in the proximity of water points.

5.~~~~ Human-animal conflict: The water points are for both human and
animal water consumption, thus creating conflict between the quantity of
water available to both and create potential health risk from water
contamination.

6.~~~~ Violent conflict: Particular conditions about the water points
create conflict between members of the community, groups of the same
community or between the local community and other communities (for
example pastoralists from elsewhere transiting in the community).

Although the environmental conditions vary, especially from the drier
southern communities in Virei compared to the wetter ones in Bibala, the
prevalence and severity of these water-related problems strongly depend
on the specific socio-economic conditions of each community. In fact,
the natural water availability is mostly related to the presence of
larger \emph{mulola} able to retain water in the sandy riverbed for
relatively long periods, albeit particular conditions apply to places
like Nascente, which stands next to a water spring (the term
``nascente'' is, in fact, the Portuguese for ``water spring''). The main
differences in water availability between the communities are determined
by the socio-economic access, which makes the better served communities
(e.g. with more wells) more resilient to hydroclimatic water stress.
This condition is reflected in the themes ``Few water points'',
``Difficult access'' and ``Unequal access''. In fact, apart from Munhino
and Tchitemo, which are the most economically developed and best served
communities with several wells along the main mulola, all the other
communities score between high and very high in these three
water-related problems (Table 1). Human-animal conflicts are quite
common in all of the communities, while violent conflicts are
particularly sever in Nascente, with the conflictual relationships with
transhumant pastoralists transiting in the community and often occupying
the only water point at the expanses of the local community (see table
S1 in supplementary material for a more extensive description of the
severity of water-related problems).

Based on the community-specific water-related problems, we identified
the main conditions to account for during the siting on a sand dam.
Although the characteristic change depending on the local conditions and
local population needs, they can be overall summarized in:

1.~~~~ Proximity to \emph{kimbos}: important to plan for a location that
can serve a large part of the community.

2.~~~~ Proximity to non-served locations: selecting sites in places
without other water sources to increase the coverage, thus promoting
equal access to water.

3.~~~~ Proximity to roads: To allow easy access for transporting
materials mechanized vehicles and reduce the costs of construction. It
is secondary in case the local community is willing to open the way by
clearing the vegetation and/or self-construct the dams..

4.~~~~ Proximity to strategic places: for example located in pasture
areas if the purpose is to serve animals or close to arable land or
community farms in case of agricultural use.

5.~~~~ Proximity to transhumance routes: Where mobile pastoralism is the
main characterizing conditions of local communities, planning the sand
dams in areas frequently used by herders during seasonal mobilizations
can help them save time when fetching water points and increase the
chance of survivor for their cattle.

Ideally, these socio-economic conditions would be integrated into the
quantitative multicriteria analysis to refine the selection of suitable
points. However, the generic problem of data scarcity in SSA, worsened
by the long and recent civil war history, together with the intrinsic
nature of mobile pastoralists make these data impossible to quantify
statistically and even more so at the spatial level. For example,
parameters like ``Proximity to \emph{kimbos}'' is hardly quantifiable
because \emph{kimbos} are temporary construction made with locally
available material (mostly wood, bushes and mud) and they change place
depending on pasture availability and informal clan rules managing the
distribution of the land within the community. For these reasons, the
abductive approach proposed in this study provides a viable way to
integrate local perspective and necessities with top-down biophysical
conditions for sand dams suitability.

\textbf{3.3 Final set of suitable sites}

The final set of suitable sites include 16 locations spread across the
10 communities, with distribution depending on the considered
water-related problems (Figure 5).

The distribution of the final set of chosen suitable sites is uneven and
was determined by the severity of the water-related problems in each
community, the presence and direction of transhumance fluxes and the
general trade-off between the need of this technology and potential
other water solutions. First, not all the communities present
biophysically suitable sites for sand dams construction strictly within
their territory. This is for example the case of Tchitemo and Nascente.
However, there are other water solutions which could be more fit for
their conditions and resources. For example Nascente has a water spring
that could be better managed and, to solve their high human-animal and
violent conflict problems, some other animal-specific water points could
be used -- for example \emph{chimpacas}, which are the local water
harvesting micro-basins for animal use. On the other hand, a larger
number of suitable points were selected in the Virei communities of
Tchicueya and Sayona (four in each community), because there is no other
better suited water harvesting technology in such an arid environment --
apart from flood wells built along the sandy rivers, which serve to use
the water harvested in the sand. Other suitable points are located in
strategic areas to serve both the local communities and the transhumant
herders passing by during the seasonal \emph{caminho}. This is, for
example, the case of SD4, which is located in the northern part of
Tchicueya and it serves both some local \emph{kimbos} and the other
herders coming from the south and directed to Bibala area.

The abductive approach has demonstrated to pragmatically deliver the
identification of a fair number of suitable points integrating both
scientific and indigenous knowledge. In some occasions, allowing the
community to freely suggest suitable suites was a successful strategy.
For example in the SD12 site in Munhino, we let the key informants (a
group of elder herders) guide us to a suitable location based on their
experience and knowledge of the place. They first led us to a
not-functioning sand dams, which was a sign that they understand the
type of water harvesting technology that they were called to support us
to identify. They also asked for the rehabilitation of the
not-functioning dam, because it used to be a key water point for the
cattle. After then, they took us to a point near a biophysically
suitable one, which represents the ideal combination of bottom-up and
top-down approaches to site identification. However, not always
communities' indications provided useful insights. In other occasions,
the points suggested by the community were in areas of larger
\emph{mulola} were there is high water accumulation in the sand, but the
dimensions of the \emph{mulola} were often too large for the type of
small sand dams assessed here, or there were no rocky banks. Also most
of these larger \emph{mulola} are subject to floods during intense
rainfall (which was noted during the focus group discussions and
confirmed during field inspections), therefore possibly not suited for
sand dams. This indicated that the bottom-up approach relying on
indigenous knowledge is much more insightful when communities are
familiar with (or well trained and informed on) the technology and they
had seen or used it in the past. This is particularly important
considering that, regardless of the methodology for sand dams siting
that is followed, there is still a significant geomorphological and
geophysical assessment work to be performed -- increasing the costs to
refine the top-down selection.

\textbf{Discussions}

\textbf{4.1 Integrated large-scale suitability analysis}

\textbf{}The mixed-methods approach, presented in this study for sand
dams siting, represent a valuable methodological contribution for a
broader set of water harvesting practices. In fact, the range of
integration of biophysical and socio-economic assessments is still
largely underexplored, although widely recommended (Ammar et al., 2016;
Bulcock and Jewitt, 2013; Castelli et al., 2022), and most water
harvesting siting approaches are relying only on biophysical MCDA. The
limitations of such top-down MCDA approaches are multiple. First, MCDA
is very sensitive to the input data, which are often scares or have
inadequate resolution in most drylands, and particularly in Angola
(Amado et al., 2020). This affects the quality of the results, requiring
a careful validation or, preferably, an integration with local analysis.
For example, Forzieri et al. (2008), Fuentes and Vervoort (2020) and
Vishwakarma et al. (2021), among others, developed analyses for similar
infrastructures based on~engineering and technical perspectives,
recognized their limitations when the results were validated, and
stressed the importance of incorporating socio-economic aspects related
to local and regional dynamics, which are difficult to measure without
an interdisciplinary perspective. Bulcock and Jewitt (2013) concluded
that the existing guidelines for determining suitable locations for
water harvesting base on biophysical MCDA tend to represent only the
optimal conditions, claiming that complementing the assessment with a
local participatory approach, like the one presented in this study, can
be instrumental.

In fact, especially when imbedded in broader water resilience strategies
for small communities, following top-down approaches fail to consider
knowledge and motivation to adopt the proposed technologies, as well as
other political and socio-economic local circumstances that influence
adaptation choices (Grum et al., 2016, Van Aalst et al., 2008). debating
with the communities and facilitating their internal discussions from
the early siting phase can increase local awareness about long-term
water supply needs and water harvesting options (Rajabu, 2005; Everard,
2019) and allow the researchers to incorporate useful criteria to siting
modeling. This has resulted in increased integration of biophysical and
sopcio-economic components in suitability analysis at different scales,
from local to national and even global (Al-Adamat et al., 2012;
Piemontese et al., 2022, 2020), although the depth of integration
between the social and the ecological components varies (Ammar et al.,
2016). These integrated assessments often rely on indicators, which
provide a quantitative and scalable measure. For example, Piemontese et.
al. proposed a social-ecological systems approach for a global
suitability of water harvesting using both biophysical and
socio-economic indicators. At the local scale, Lasage and Verburg (2015)
validate and recommend a similar mix of biophysical and socioeconomic
conditions to be considered in order to focus on pertinent
characteristics and to ensure an integrated approach when evaluating
water harvesting investments.

Our contribution adds to this literature by providing a practical
framework to explicitly integrating local knowledge, which is a major
frontier in integrated assessments (Al-Adamat et al., 2012; Hart and
Mouton, 2005; Robinson et al., 2016). Also, particularly relevant is the
application to sand dams, for which such integrated approach was still
missing, although highly recommended by experts (Castelli et al., 2022).
Our approach thus contributes to expanding the frontiers of knowledge
integration on water~ harvesting implementations and providing a
specific guidance on sand dams large-scale siting to increase the spread
and the applicability of such a promising technology.

\textbf{4.2 Insights into sand dams viability and overall fit in Namibe}

To support the rapid interest and prospective investments on sand dams
in Angola (World Bank, 2022), Serrat-Capdevila et al. (2022) have
recently ~investigated the potential for constructing sand dams in the
area, although without providing a specific potential location for sand
dams construction. Our final results (the 16 locations assessed with the
integrated approach) can thus provide concrete support to investments on
sand dams in the area, supporting local water development projects with
both our quantitative and qualitative results. In fact, another key
finding is that colonial sand dams have been largely used by indigenous
people and are considered an asset for the communities (e.g. the
colonial dam of Panguelo is the only reliable water point of the
community), as opposed to other colonial structures like wells and
houses who fell into disrepair or were even vandalized by local herders
to reconquer their territory or because of their symbolic meaning of
colonial power (Ramazzotti, 1991). The fact that sand dams became
embedded in local water use traditions represent thus a peculiar
situation, in contrast with most experiences also from larger dams (Liu
et al., 2019). This provides new evidence on the viability of sand dams,
not only as a biophysically suitable, but also socio-economically and
culturally fit technology, encouraging the development of small,
distributed and community driven dams for increasing water security in
Namibe.{}

Apart from the territory of the 10 communities (municipalities of Bibala
and Virei), our analysis can be used to support other pastoral
communities, especially considering that water scarcity is a problem
that affects people beyond community's boundaries given the mobile
nature of pastural traditions and the increasing climate change-driven
migratory trends (Neumann et al., 2015; Sterzel et al., 2014). With our
province-scale analysis, other communities could benefit from sand dams
construction as adaptation and mitigation strategy to address the
increasing impact of climate change and water scarcity. The 2106
biophysically suitable points in the whole province might seem quite a
large number considering the time and resources required for a
complementary in-field socio-economic assessment. However, other
large-scale studies provide a comparable number of suitable points,
recommending the integration of local participatory work or at least a
local cross-check/validation of the approach, though mostly leaving the
in-field part unmet for technical, costs and/or time constraints (Ammar
et al., 2016). Nonetheless, we recommend considering our biophysically
suitable points as a first selection to be complemented by participatory
work, given the possible biases related to the small set of parameters
used in the MCDA conceived for a complementary community engagement.

\textbf{Challenges and way forward}

The process of developing a mix-method approach and the extensive field
experience open up reflections on a range of challenges and
opportunities related to a) the ideal balance between data-driven,
top-down biophysical analysis and qualitative participatory research, b)
the usefulness and necessity of participatory work beyond the
best-siting phase and finally c) a more generic reflection on the fit
and implications of sand dams development for frugal, semi-nomadic
pastoralists.

\emph{Knowledge integration}

A critical part of the methodological process was identifying the level
of integration between the biophysical assessment and the participatory
phase. The criteria for the biophysical analysis were kept to a minimum
set of variables, which rendered a coarse first identification of
points. Many of these points are likely not overall suitable given the
further biophysical requirement that were not accounted in this work for
lack of good quality data, but which could be integrated in future
analysis. For example, with a higher resolution DEM (i.e.5m or less) it
would be possible to estimate the riverbanks height and slope and the
throwback area. However, over-reliance on top-down MCDA could lead to
downgrading the role of participatory analysis, which should remain
central to guarantee a proper inclusion of local stakeholders in the
study and the follow up decision process on the implementation (Kallis
et al., 2006). Thus, a methodological dilemma remains: Is it more
appropriate to include coarse quantitative estimates of biophysical
variables or rely on less uncertain estimates from fieldwork? The
advantage of the first option is to provide a quick and inexpensive set
of suitable points for guiding a more focused in-field inspection.
However, the accuracy of the findings might be lower compared to a more
extensive in-field participatory phase. The balance between these two
approaches is a common struggle that require a good knowledge of the
place and careful planning. We suggest tuning the balance between the
two research phases depending on spatial data availability, knowledge of
the place and level of community engagement.

\emph{Participatory research beyond best-siting of sand dams}

Beyond the scope of sand dam siting, the participatory engagement helped
convening local communities to raise awareness, debate about their needs
and experiences and uncover essential matters to take into account for
water resources development projects. In this sense, the participatory
research approach described here can be useful to a wider research
community, since small scale interventions that engage communities have
better chances to achieve more engagement in the construction process,
ownership of the outcomes, and even foster diffusion of innovations
(Mansuri and Rao, 2004; Mulligan et al., 2011). Furthermore, the
discussions on why some water points and structures fail or are
vandalised was valuable to debate about the importance of maintenance of
infrastructure and care within beneficial communities. The focus group
discussions promoted a debate on technical matters that helps changing
the focus from covering immediate water security needs to developing a
longer-term view from the communities, with more emphasis on medium and
longer-term needs and more debate on operation and maintenance of the
introduced infrastructures (Garfias Royo et al., 2018). More generally,
these engagements, facilitated by NGOs, can promote the creation of
Water and Sanitation Groups (Grupos de Água e Saneamento), which are
local consumer associations or councils, and the implementation of more
or less organized models of community water management like the MOGECA
model, which is active in some rural municipalities of Angola, but not
enough widespread in the south (Baptista and Rall, 2021). A lesson
learnt in this work, and highly recommended in other water resources
development projects in similar contexts, is the need to guide
communities towards a deep understanding of the long term sustainability
approach of water harvesting infrastructure and shift the focus from
short-term to long term benefits, with the corresponding change in
expectations, approach and mentality.{}

\emph{Fixed water resources for mobile pastoralists}

A final reflection concerns the epistemological approach to water
resources development. It is clear that climate change is increasing
resources variability and uncertainty, with negative consequences on
local communities. Nonetheless, the solutions proposed by outsiders
development projects must acknowledge the diverse universe of point of
views and approaches to face such changes and support a really
sustainable development (Pisor et al., 2022). The model, although
generally positive, of increasing water points and introduce effective
water harvesting infrastructure learning from elsewhere experience,
should critically reflect on the compatibility with existing mechanism
used by local communities, even outside of the scientific literature and
debate. Some recent studies on pastoralism and vulnerability suggest
mobility as an adaptation of pastoralist to uncertainty and variability,
which constitute the very nature of dryland livelihoods. Mobility is
seen as an adaptation strategy, which make pastoralists resilient to
climatic and other changes (Turner et al., 2014). Under this point of
view, supporting such communities with fixed water points might
introduce a factor of disturbance in pastural mobility, with the
possible consequence of fixing communities because of the initial
benefit in availability and accessibility of water, but which might
disrupt the mobile arrangement leading to a long term reduction in
resilience (Scoons, 1995). These dynamics needs to be further
investigated in relation to water harvesting development projects,
requiring a dedicated data collection and participatory engagement.

\textbf{Conclusions}

In this work, we introduced a mixed-method approach combining top-down
biophysical suitability with bottom-up participatory research to
approach large-scale siting of sand dams. While the introduced
analytical framework can be used as a general approach to any
integrated, large-scale water harvesting siting, the application to sand
dams in drylands Angola helped revealing the presence of existing sand
dams, which were not reported in previous scientific literature. The
existing sand dams well match the potential site identified with the
biophysical assessment, confirming the validity of the proposed
approach. Overall, the approach has proved to successfully guide the
quick identification of 16 potential sites matching the socio-economic
requirements investigated during the participatory phase and validated
with the local population. However, challenges do exist. The
participatory approach should be central, requiring a careful knowledge
integration and constant engagement of local people, which is key for
ensuring sustainability beyond the siting phase and the project
timeframe. Being this research embedded in a cooperation and development
project, a follow-up of the case study could monitor and report on the
actual functioning and beneficial use of the sand dams to be built in
the selected sites, complementing the scarce literature on the complete
life-cycle of sand dams implementations, especially relevant for
data-scarce regions like dryland Angola.

\textbf{Acknowledgments}

''The work was developed within the project ``TransAgua: valorização das
boas práticas dos pastores transumantes em gestão dos recursos hídricos
e adaptação às mudanças climáticas'', funded within the FRESAN
(FED/2017/389-710 - FORTALECIMENTO DA RESILIÊNCIA E DA SEGURANÇA
ALIMENTAR E NUTRICIONAL EM ANGOLA''), by the Istituto Camões --
Instituto da Cooperação e da Língua).''

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Table 1. Severity score of water-related problems for the 10
communities.

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