Detecting the Response of Irrigation Water Management to Climate by Remote Sensing Monitoring of Evapotranspiration

摘要

This study on a large irrigation scheme in Morocco addressed a two-fold question: (a) does the local water management authority adapt water releases to atmospheric water demand ET0-P? and (b) does crop actual evapotranspiration respond to inter- and intra-annual variability in water releases? We have evaluated the inter-annual variability of ET0-P during the period 1992−2017 and compared its anomalies (i.e., deviations from average) with anomalies in annual water release. Overall, it appeared that anomalies in water release were consistent with anomalies in ET0-P. The actual evapotranspiration (ETa) was estimated using a time series of multi-spectral satellite image data by applying the Surface Energy Balance (SEBAL) algorithm in a dry, wet, and reference year. We have determined the quartiles of the distribution of the ET0-P time series to identify these three years. The dry year was 2015−2016, the wet year was 2014−2015, and the reference (median of ET0-P) was 2013−2014. Finally, we compared seasonal and annual anomalies in ET0-P, ETa and release, Wd of irrigation water. In the period 1992−2017, the relative anomalies in ET0-P and Wd were similar only in 2000−2001 and 2015−2016. The analysis of anomalies in water inflow and stocks confirmed the response in increased Wd following wet years with higher inflow and replenishment of the reservoir. The response of crop water use to the irrigation water supply was evaluated by comparing anomalies in the ratio of actual to maximum ET, i.e., ETa/ETc with anomalies in Wd. As regards the Sidi Bennour, Faregh, and Gharbia districts, the relative anomalies in ETa/ETc and Wd were consistent, i.e., they had the same sign and comparable magnitude. Overall, the study shows that water delivery Wd responds to inter-annual variability in the pre-irrigation season water inflows into the reservoirs, rather than in ET0-P. On the other hand, actual crop water use, i.e., ETa/ETc, does respond to inter- and intra-annual variability in Wd. This evidence suggests that there is scope for adaptive water management based on a flexible adaptation of water release to inter- and intra-annual variability in water demand.