How Changes in Harvested Area Impacts the Actual Evapotranspiration of Croplands Using Optical Remote Sensing

Associated with drought, the increased agricultural water consumption in arid and semi-arid regions has caused water competition among water users and worsened water scarcity and food security. Hence, there is an imminent need for accurate and reliable estimation of actual evapotranspiration (ETa) for large scales, as a key component of the water cycle due to its critical role in determining crop water requirement. This work aims to investigate the impact of changes in harvested area (HA) over time on ETa estimates using remote sensing (RS) in the Zayandehrud River Basin, Iran, where croplands are highly dependent on irrigation and strongly influenced by aridity and recurring drought events. RS provides a dependable basis for ETa quantification across large areas, particularly regions where lack of ground data hampers ETa estimation. The efficient RS data handling is of importance. In this regard, Google Earth Engine (GEE), a cloud-based open-access platform for accessing and analysing the RS data, was used to derive ETa and HA. The Vegetation Index-based ETa (ET-VI) approach is one of the RS-based methods which combines a vegetation index as a proxy of crop factor and reference ET (ET0) to estimate ETa. We calculated ET-VI as a tool to monitor agricultural water consumption and drought over croplands (2000-2019). Since reducing crop area is a common strategy employed by farmers against drought, HA tends to show considerable inter-annual variability, particularly in semi-arid regions, where drought is a major factor affecting rainfed and irrigated agriculture. Shrinkage of HA during water-stressed periods often leads to an ETa underestimation when HA changes are ignored in ETa estimation (i.e., HA is assumed static for ETa calculation). To assess the effect of cropping patterns’ inter-annual change on the annual ETa, annual maximum Normalized Difference Vegetation Index layers were derived. Analyses of HA and ET-VI showed that inter-annual variability in cropland extent affects ETa considerably, therefore using static cropland extent is not recommended in drought studies. ETa remained less variable while cropped areas changed in response to dry years. This means that drought has forced farmers to use the limited available water on a smaller area to cope with drought and safeguard reliable crop production. The average difference between ET-VI estimated based on static and dynamic HA was 221.7 mm per annum in our study area. Our findings highlight the necessity of incorporating both cropped areas and ETa rates in water management and drought monitoring of croplands. Our analysis offers insights into the capability and suitability of RS-based ETa as an efficient and quick tool for understanding spatio-temporal variability of ETa across croplands and monitoring water resources. Future research should evaluate the potential of high-resolution sensors with frequent return in the ETa derivation to monitor drought, vegetation health, and water consumption at different time scales, and whether they can improve the accuracy of drought mapping and monitoring.