Uncertainty in Future Projections of Water Deficit Droughts Based on
Evapotranspiration Methods in the Indian Subcontinent
Abstract
The choice of reference evapotranspiration (ETo) estimation methods and
general circulation model (GCM) are crucial for projecting water deficit
under a changing climate. Standardized Precipitation Evapotranspiration
Index (SPEI) derived from water deficit also varies with the choice of
GCM and ETo estimation methods. In this study a variance-based global
sensitivity analysis was used to estimate relative sensitivity of
projected changes in future water deficit (P-ETo) and SPEI to the choice
of GCM and ETo estimation methods over parts of the Indian subcontinent.
For evaluating the change in water deficit and droughts, 7 GCMs and 11
ETo methods were analyzed for two distinct periods i.e. 2030-2060 and
2070-2100 compared to the baseline (1951-1980). The 11 ETo methods were
grouped into 4 major categories namely based on temperature, radiation,
mass transfer and combination methods. Moreover, based on the ETo
categories, a non-parametric Mann-Whitney was performed to quantify
robust changes under a warming climate. Results show that changes in
future water deficit and droughts varies with regions and seasons.
Overall, changes in water deficit droughts are more inclined to the
choice of ETo method, while the GCM-ETo interaction effects are more
prominent in some regions. Results also showed that within an individual
ETo category, individual ETo methods do not necessarily agree on the
magnitude/direction of change in projecting water deficit and SPEI for
future conditions. This has important implications towards selection of
appropriate ETo estimation for drought analysis in data scarce regions
under a changing climate. Results of this study indicate, the role of
proper ensemble formation of GCMs and ETo estimation methods based on
seasons and regions, to develop a robust range of future conditions for
water resources planning.