Structured hydrological analysis for targeting fallow evaporation to improve water productivity at the irrigation system level

Abstract. This paper provides results of an application of a holistic systematic approach of water accounting using remote sensing and GIS coupled with ground water modeling to evaluate water saving options by tracking non-beneficial evaporation in the Liuyuankou Irrigation System (LIS) of China. Groundwater rise is a major issue in the LIS, where groundwater levels have risen alarmingly close to the ground surface (within 1 m) near the Yellow River. The lumped water balance analysis showed high fallow evaporation losses and which need to be reduced for improving water productivity.

The seasonal actual evapotranspiration (ET_s) was estimated by applying the SEBAL algorithm for eighteen NOAA AVHRR-12 images over the year of 1990–1991. This analysis was aided by the unsupervised land use classification applied to two Landsat 5 TM images of the study area. SEBAL results confirmed that a significant amount (116.7 MCM) of water can be saved by reducing ET_s from fallow land which will result in improved water productivity at the irrigation system. The water accounting indicator (for the analysis period) shows that the process fraction per unit of depleted water (PF_depleted) is 0.52 for LIS, meaning that 52% of the depleted water is consumed by agricultural crops and 48% is lost through non-process depletion.

Finally, the groundwater modeling was applied to simulate three land use and water management interventions to assess their effectiveness for both water savings and impact on the groundwater in LIS. MODFLOW's Zone Budget code calculates the groundwater budget of user-specified subregions, the exchange of flows between subregions and also calculates a volumetric water budget for the entire model at the end of each time step. The simulation results showed that fallow evaporation could be reduced between 14.2% (25.51 MCM) and 45.3% (81.36 MCM) by interventions such as canal lining and ground water pumping. The reduction in non-beneficial ET_s volumes would mean that more water would be available for other uses and it would allow the introduction of more surface water supplies in the area through improved water management strategies. This will ultimately lead to improved water productivity of the LIS system.