An Innovative Modeling and Measurement Approach to Improve Rice Water Use Efficiency in California

Abstract:

California is amongst the top rice producing states in the USA, and more than 95 percent of California’s rice is grown in the Sacramento Valley. Based on older literature, the rice water requirement (ET_c), ranges between 914 and 1,100 mm. In this study, the actual rice water requirement was measured using the residual of the energy balance method over three paddy rice fields during 2011-2013 seasons in the Sacramento Valley. Net radiation and ground heat flux density were measured, and both eddy covariance (EC) and the surface renewal (SR) technique were employed to determine the sensible heat flux density. The surface renewal method uses high frequency temperature measurements from fine wire thermocouples above the canopy. Mean amplitude and duration of the ramps over half hour periods were determined using a structure function and the characteristics are employed to estimate the direction and magnitude of sensible heat flux using the ratio of the amplitude to the ramp duration as the change in temperature per unit time and the volumetric heat capacity of the air to estimate the magnitude of the heat flux. In the study, 76.2 mm diameter chromel-constantan thermocouples were used to measure high frequency temperature at 10 Hz.

The results indicate that there is considerable variability in rice water use both spatially and temporally. The average three-year measured ET of the experimental fields located in Butte County was 734 and 725 mm; and in Colusa County was 771 mm. A typical crop coefficient (K_c) curve was derived from the measured ET_c and reference ET (ET_o) data. Spatial estimates of monthly climate data from the Sacramento Valley were used to calculate monthly mean ET_o, and smooth curve fits of the monthly data gave estimates of typical daily ET_o. The daily ET_c was calculated as the product of ET_o and K_c, and seasonal ET_c was calculated by summing the daily ET_c values. The results reveal that the seasonal rice ET_c was less than earlier estimates. Surface renewal equipment is relatively inexpensive and it provides rice growers with remote monitoring of water consumption by tracking ET_c of the paddy fields; and hence assists them to improve rice water use efficiency. The information provided here is useful for determining volumes of water for water transfers and insuring adequate water supplies for optimal production.