A novel method to quantify consumed fractions and non-consumptive use of irrigation water: Application to the Indus Basin Irrigation System of Pakistan

Increasing irrigation efficiencies remains the focus of numerous efforts to mitigate water scarcity. In reality, higher local efficiencies do often not reduce water scarcity, but instead cause a redistribution of water flows when the entire irrigation scheme or river basin is considered. Insufficient understanding of consumed fractions and non-consumptive use (i.e. return flows) have led to ineffective, or even harmful, water conservation measures. In this paper, we demonstrate a novel method for spatial quantification of the Consumed Fraction (CF) of withdrawn irrigation water based on satellite remote sensing and the Budyko Hypothesis. This method was applied to evaluate consumption of irrigation water (ETblue), total water supply, and non-consumptive use across the Indus Basin Irrigation System (IBIS) of Pakistan. An average ETblue of 707 mm/yr from irrigated cropland was found for 2004–2012, with values per Canal Command Area (CCA) varying from 421 mm/yr to 1011 mm/yr. Although canal supply (662 mm/yr on average) in most CCAs was largely sufficient to sustain ETblue, a similar volume of additional pumping (690 mm/yr) was required to comply with hydro-climatological principles prescribed by Budyko theory. CF values between 0.38 and 0.66 were computed at CCA level, with an average value of 0.52. Co-occurrence of relatively low CF values, high additional water supply, and long-term canal diversions similar to ETblue, implies that the IBIS is characterized by extensive reuse of non-consumed flows within CCAs. In addition, the notably higher CF of 0.71−0.93 of the full IBIS indicates that return flow reuse between CCAs cannot be neglected. These conclusions imply that the IBIS network of irrigators is adapted to extensively recover and reuse drainage flows on different spatial scales. Water saving and efficiency enhancement measures should therefore be implemented with great caution. By relying on globally available satellite products and limited additional data, this novel method to determine Consumed Fractions and non-consumed flows can support policy makers worldwide to make irrigation systems more efficient without detriment to downstream users.