Bruce Linquist, UCCE Rice Specialist, Dept. of Plant Sciences, UC Davis

The goal of this project is to identify opportunities to conserve water in California rice systems. Objectives for 2018 were:

Percolation and seepage losses

Direct measurements of percolation losses were made in eight fields spread throughout the Sacramento Valley in the 2017 growing season. Percolation rings were installed at three locations in each field, and the weekly change in water height inside the rings was measured. Percolation rates ranged from 0.02 inches to 2.8 inches per season, assuming 115 flooded days. The average percolation rate across all sites was about 1 inch per season. Percolation rates could not be predicted by any soil properties. Unsaturated zones tended to have higher percolation rates, although they were small compared to total irrigation input.

Lateral seepage was measured directly at 50 locations spread across six rice fields during the 2017 growing season.  Lateral seepage varied greatly but was universally low compared to irrigation inputs onto rice fields. No correlation was found between lateral seepage rates and soil texture, levee width, or field water height. As expected, however, lateral seepage rates varied depending on whether the field was bordered by a drainage ditch, fallow field, flooded field, or supply canal.

Similarly, lateral seepage rates were affected by the water height on the other side of the levee. Seepage out of the field was observed at locations where the field was bordered by a supply canal.  Lateral seepage from both field and supply canal would flow into levees if they were unsaturated.

Water balance

A complete water balance was calculated for three commercial rice fields in the Sacramento Valley. Irrigation inputs were measured by Reclamation District 108, Reclamation District 1004, and the Richvale Irrigation District. Rainfall was measured at the nearest CIMIS weather station. Evapotranspiration was calculated from CIMIS data and recently developed crop coefficients for California rice. Tailwater drainage was measured with an outlet weir and data loggers that recorded the height of water above the weir crest.

Evapotranspiration accounted for approximately one-half to two-thirds of the applied water, with tailwater drainage representing the second largest outflow. Only 90% of water inputs on average were accounted for as water outputs across all sites. The water balance approach may overestimate cumulative percolation and lateral seepage, while direct measurements may underestimate them. The actual contribution of percolation and lateral seepage likely lies somewhere in between.

Based on this research, the minimum requirement to grow rice with no-spill management is in the range of 41–43 inches. However, it must be stressed that any particular field may have factors that significantly alter its water balance and its irrigation water requirement.

Publications

A manuscript exploring the magnitude and variability of lateral seepage rates in California rice fields, and the influence of soil properties and hydrologic conditions on lateral seepage rates has been submitted to the Journal of Hydrology. A second manuscript is in progress and will be submitted to the journal Agricultural Water Management.

A fact sheet has been developed and is available on the website rice.ucanr.edu. This is a valuable resource for growers, irrigation district managers, and state water officials.