The impact of expanding flooded land area on the annual evaporation of rice

• Annual evaporation of rice grown in California decreased nearly 15% over 6 years.
• The oasis effect enhanced evaporation when the field was smallest.
• The rice evaporated near potential rates when the field was largest.

The amount of published data on annual evaporation on rice remains extremely limited despite the role of rice as a key food source. We report on six years of rice evaporation measurements, based on the eddy covariance method. This rice was cultivated in the hot dry climate of California, where water is a scarce and precious resource. During the first year, we found that rice evaporation exceeded potential evaporation rates and summed to 1155 mm y−1. In following years, we found that annual evaporation decreased yearly, yielding a 15% reduction (to 982 mm y−1) by the sixth year. The remainder of the paper examined the how and why of this unexpected decreasing trend in annual evaporation occurred.We inspected trends in variations in biophysical variables (net radiation, temperature, leaf area index) associated with evaporation and potential biases in the flux measurements using energy balance closure and co-spectral analysis. None of the biophysical variables varied enough to explain this observation. What did change was the area of rice, and nearby flooded wetlands. During the first year, the flooded rice field was less than 1 km2 in area and was relatively isolated. This situation promoted an ‘oasis effect’ that enabled warm dry air to be entrained from across the top of the planetary boundary layer; this contention was supported by the co-spectral analysis and analysis with a coupled surface energy balance-planetary boundary layer model. By the sixth year of this project, the area of flooded rice and wetlands approached 6 km2, a horizontal scale that seemed to inhibit the ‘oasis effect’. We conclude that the amount of water used, on an area basis, by wetland restoration projects will depend upon the spatial extent of these projects.