Spatial and Temporal Dynamics of Salinity in California Rice Systems.

The scarcity of quality irrigation water is a key issue facing California rice growers, forcing many to adopt water management systems that may result in increased salinity and yield reductions.  Field water salinity concentrations have been shown to vary depending on water management; however, the distribution and build-up of dissolved salts is unclear.  This study aims to elucidate the spatial and temporal salinity dynamics in California rice systems, and to assess current yield thresholds.  In this two-year study, water and soil salinity concentrations of 11 field sites were monitored weekly, with 9 sampling points being established in the top, middle and bottom basins of each field.  There was a consistent water salinity pattern throughout all fields: an increase in water salinity occurring during week 1 to week 7, ranging from 0.39 to 6.06 dS/m, and being greatest further from the irrigation inlet and in positions with high soil salinity.  A model to predict water salinity within a field was developed.  Averaged over an entire growing season, position within a field contributed to roughly 79% of the explained variation in the model (R²=0.59), while soil salinity contributed to roughly 21%. Soil salinity significantly influenced water salinity, especially early in the season; however, it was a poor metric for predicting yield loss.  For water salinity, the threshold concentration when yield reductions occurred was 0.88 dS/m, well below the previous report.  These results showcase the ability to predict water salinity in a rice field with few parameters while highlighting the importance of focusing on field water salinity as the salinity metric for rice systems.