Evaluation of Crop Water Use and Rice Yield Using Remote Sensing and AquaCrop Model for Three Irrigation Schemes in Sri Lanka

Abstract: With a changing climate and an increased competition over water resources for agricultural irrigation, the need to improve crop water productivity using time and cost-efficient methodologies have become critically important. The Malwathu Oya river basin in Sri Lanka is struggling with water scarcity, which threatens food security and the income of farmers. In this study, freely available remote sensed land- and water productivity data from FAO’s WaPOR database was evaluated. The evaluation consisted of a comparison of the WaPOR data and primary collected field data using the crop water model, AquaCrop, for three irrigation schemes in the Malwathu Oya river basin. Additionally, the spatio-temporal variability in crop water use within and across these three irrigation schemes was assessed using indicators derived from the WaPOR portal. The evaluation was conducted for the main cultivation season, called Maha, between 2010 and 2021.  The WaPOR and AquaCrop actual evapotranspiration (ETa) values were found to be in relatively good agreement (312–537 and 400–465 mm respectively). WaPOR yield values (2.5–2.9 ton/ha) were however lower compared to the AquaCrop simulated yield values and historical yield data (4.6–5.7 and 4.4–5.6 ton/ha respectively). Difference in calculation methodology, possible sources of error in WaPOR conversion calculations and limitations in accuracy caused by cloud coverage when collecting satellite data could be explanations for this. Prior knowledge and accurate allocation of the crop type and parameters used in conversion calculations in WaPOR is therefore of significant influence. From the spatio-temporal variation assessment with WaPOR indicators, a fair uniformity of the water distribution within the irrigation schemes was shown (CV 11–19 %). The beneficial water use (BWU) in the irrigation schemes showed lower values (50–90 % allocated to T) for years when the available water amount was higher, which could be explained by the higher rate of water lost through soil evaporation. Crop water productivity (CWP) values showed higher values (about 0.70 kgDM/m3) when the available water amount was higher, indicating that yield production is sensitive to water-scarce environments. Applying a yield boundary function, representing the best attainable yield in relation to water resource, showed that there is potential to achieve the same yield with less amount of water. There are thus possibilities for improved water productivity in the three irrigation schemes investigated. For future research it is recommended to perform a sensitivity analysis for WaPOR and ground truth with yield data to obtain a better understanding of potential limitations. To obtain more precise site descriptions it is also recommended to ground truth AquaCrop with yield and soil data.