The contribution of percolation to water balances in water-seeded rice systems

Abstract

Rice (Oryza sativa) has one of highest applied water footprints of any crop, due in many cases to high percolation and lateral seepage rates in flooded rice fields. Better understanding the magnitude and variability of these subsurface water flows and their contribution to the rice field water balance is critical for efforts to reduce the water footprint of rice production and to limit the transport of pollutants to surface water and groundwater. Percolation was directly measured in eight direct-seeded California rice fields that ranged from 20 to 61% clay and a complete water balance was developed for three of these fields. For these latter fields, directly measured percolation rates were compared to percolation calculated with Darcy’s law, and combined percolation and lateral seepage calculated as the residual of a water balance was compared to directly measured values. Across eight fields, cumulative percolation over the growing season ranged from 0.04 to 6.95 cm season−1. The mean cumulative percolation for the three water balance fields was 2.1 cm based on direct measurements compared to 3.2 cm based on Darcy’s law calculations. Combined percolation and lateral seepage calculated as the residual term of a water balance for the three fields was 17.1 cm, compared to 4.4 cm based on direct measurements, corresponding to 13.2 % and 3.4 % of water inputs, respectively (inputs were 98–99 % from irrigation). Based on these results, water management strategies that remove floodwater (e.g. alternate wetting and drying) would have limited potential to reduce water inputs in California rice production (0.7–2.7 cm season-1). Furthermore, using the most conservative (largest) estimates for each component of the water balance, we conclude that the average water requirement for California rice fields is approximately 108 cm season-1.