Efficient water management in agricultural crops is necessary to increase productivity and adapt to climate change. Evapotranspiration (ET) data are key to determine water requirements of crops and set efficient irrigation schedules. Estimating ET at regional scale (for example, in irrigation districts) is a technically complex task that has been tackled by using data acquired by remote sensors on satellites that can be validated with scaled up field measurements when area sources are matched. Energy and matter flux measurements using the eddy covariance (EC) technique are challenging due to balance closure issues, claimed to be due to the different footprints of the energy-balance components. We describe net radiometer footprints in terms of the sun-sensor geometry to characterize the bidirectional distribution functions of albedo and thermal emissions. In this context, we describe a one-parameter model of the components of net radiation that can be calibrated with a single data point. The model was validated in an experiment with five agricultural crops at Valle del Yaqui, in Sonora, Mexico, using different sun-sensor geometry configurations. The results from the experimental fits are satisfactory (R2 > 0.99) and support the use of the model for albedo and radiative (surface) temperature in order to estimate net radiation. The analysis of the implications regarding a mismatch among footprints of the components of the energy balance showed that net radiometer fluxes are most of the time overestimated implying that the closure problem could be solved using similar footprint as aerodynamic components of the energy balance.

Forests are under major pressures due to contemporary land-use, which creates mosaics of stand-stage development that follow different successional paths, that imply ecosystem complexity. The interplay of carbon and water dynamics across succession involves physical and biological interactions that shape net ecosystem production (NEP) and water use efficiency. Here we present 13 years-site of eddy covariance data (2016-2020) from a seasonally dry tropical forest in Northwestern Mexico to elucidate the environmental controls on ecosystem fluxes and explore the interactions with changes in resource availability. Across a successional gradient, an early (9 years since abandonment) and a mid-successional (about 45 years with natural recruitment and regrowth) sites were net carbon sinks (in the order of 100 to 500 g C m-2 y-1) while an old- growth forest was a chronic net source over the 5 years studied (losing between 100 and 300 g C m-2 y-1). In contrast evapotranspiration was alike at sites and close to the precipitation input. Ecosystem water use efficiency tended to be higher at the old-growth forest site (ca. 3.0 g C m-2 /mm H2O vs. ca. 2.0 g C m-2 / mm H2O at the secondary sites). Water availability and radiation where clearly dominant environmental controls across sites, but notably vapor pressure deficit was not a controlling factor for gas exchange at the old-growth forest. Surface characteristics, canopy structure and species composition may explain differences in NEP across succession in TDF at its northernmost extent.

Soil moisture is an essential measurement to manage water and improve crop production. However, agricultural research in the Yaqui Valley (in northwestern Mexico) with extensive wheat fields (Triticum sp.) have focused on other monitoring schemes (e.g. remote sensing) with less attention to soil moisture. Most of this cultivated soil contains up to ~ 50% clay, which results in changes to soil properties from wet to dry conditions and challenges in the implementation of in-situ measurements of soil moisture. For this research, we selected a 1-ha wheat field in the Yaqui Valley representative of a typical flood irrigation system. We measured meteorological variables (ClimaVUE™50), and soil moisture for the winter crop-cycle from December 2019 to Abril 2020. Volumetric water content (VWC) was recorded from 5 to 50 cm using two TDR (SoilVUE™10), one located in the bottom of the furrow under bare conditions, and the other on the top under the vegetated condition for further integration and comparison. A Cosmic Ray Neutron Sensor (CRNS) was located alongside the meteorological sensor. The universal calibration equation was used to estimate VWC based on neutron counts. The comparison from the CRNS and the integrated TDR (5 to 50 cm) resulted in an RMSE of 0.02 m3m-3 and an r2 = 0.73. While both technologies respond to water inputs, the CRNS is a more reliable measurement during the dry-down periods when the high-clay soil cracks to the extent of 40 cm where soil is exposed to air. During this driest period, recorded VWC at 50 cm was, on average, 0.25 m3 m-3, while measurements with the CRNS was on average, 0.16 m3 m-3. Interestingly, both sensors peaked at 0.56 m3 m-3 during the flood irrigation event.