Circulation water and solute mass balance model for the rice growing area on the right bank of the lower Guadalquivir River valley

The rice-growing region on the right riverbank of the Lower Guadalquivir Marshes in Spain is a highly productive area that spans approximately 22,500 ha and involves 13 Water Users Associations (WUAs) and independent producers. Rice cultivation in this region demands around 1000 mm of flood irrigation annually at the district level, although individual fields may receive up to four times this amount due to high surface drainage and water recirculation rates. Irrigation water is sourced from the Guadalquivir estuary, influenced by tidal dynamics, where salinity represents a critical constraint, further intensified by the region's water shortages. The aim of this study is to develop a district model to explore management options that help deal with water scarcity and salinity. A ‘bucket’ mass balance model with circulation rules and capacity constraints was employed to analyze daily water and salt balances in this rice-growing area. The interaction between irrigation units required the conceptualization of the system, done in collaboration with WUAs, and structured as a mesh layout of the distribution network, where connection nodes, consisting of drains, collect return flows from irrigation units and provide reused water for irrigation. Water mass and salt concentrations were monitored at specific points within the rice-growing area, and these measurements were compared with the model outputs.

Results showed good agreement with measured values of discharge volumes and salinity. Subsequently, two groups of alternative scenarios were simulated: the first one focused on the benefits of upscaling on-farm irrigation practices and their effectiveness in saving water, while the second group showed effectiveness for minimizing salinity-related constraints. The model resulted in a useful tool to simulate different water management practices and evaluate their impact as possible improvements in the performance of the entire district.