Measuring Crop Water Use in CA Rice 00

Measuring Crop Water Use in California Rice-2000
Project Leader and Principal UC Investigators Richard L. Snyder, biometeorologist Department of Land, Air and Water Resources, University of California, Davis	The objective of this project is to develop accurate rice crop water use information for the industry, water purveyors and policy makers. Previous estimates of rice crop evapotranspiration were based on studies conducted more than 30 years ago. With new varieties and changes in the methods to estimate reference evapotranspiration, the old figures do not accurately reflect how much water a modern rice crop actually uses. An experiment was conducted in Sutter County near where Highways 99 and 70 intersect and not far from the Nicolaus California Irrigation Management Information System (CIMIS) site. The purpose of the experiment was to measure rice crop evapotranspiration and to determine crop coefficient values relative to reference evapotranspiration from the CIMIS site. The evapotranspiration rate for rice depends on the amount of energy available to vaporize the water. This is estimated using the energy balance of net radiation, water and soil heat storage, sensible heat flux and latent heat flux (evapotranspiration). Net radiation is the balance between all sources and losses of radiation by the crop and is measured with a net radiometer. Heat storage in the water and soil is measured using temperature and heat flux sensors in the water and soil. Sensible heat flux is the transfer of heat that you can sense (or measure with a thermometer) and is determined with a new technique called "surface renewal." Air temperature is measured four times per second with fine wire thermocouples. Sensible heat flux is calculated using turbulence statistics. The energy used for evapotranspiration of the water is calculated as the net radiation minus heat storage in the water and soil, minus the sensible heat transfer from the crop to the air. The total seasonal water used (31.5 inches) was about 75 percent of what has been typically reported for California (42 inches). Crop coefficient values started at about 1.30 and decreased to about 1.09 when the canopy reached about 65 percent ground cover. During the main part of the season, these values remained at about 1.09 and then dropped to about 0.90 at the end of the season. Thus preliminary indications are that current rice crop water use is less than what earlier projections indicate. Because these results are so different from earlier reports, the experiment is being repeated again in 2001 to confirm the findings.

Measuring Crop Water Use
in California Rice-2000

Project Leader and Principal UC Investigators

Richard L. Snyder, biometeorologist Department of Land, Air and Water Resources, University of California, Davis

The objective of this project is to develop accurate rice crop water use information for the industry, water purveyors and policy makers. Previous estimates of rice crop evapotranspiration were based on studies conducted more than 30 years ago. With new varieties and changes in the methods to estimate reference evapotranspiration, the old figures do not accurately reflect how much water a modern rice crop actually uses.

An experiment was conducted in Sutter County near where Highways 99 and 70 intersect and not far from the Nicolaus California Irrigation Management Information System (CIMIS) site. The purpose of the experiment was to measure rice crop evapotranspiration and to determine crop coefficient values relative to reference evapotranspiration from the CIMIS site.

29.jpg (68299 bytes) The evapotranspiration rate for rice depends on the amount of energy available to vaporize the water. This is estimated using the energy balance of net radiation, water and soil heat storage, sensible heat flux and latent heat flux (evapotranspiration). Net radiation is the balance between all sources and losses of radiation by the crop and is measured with a net radiometer. Heat storage in the water and soil is measured using temperature and heat flux sensors in the water and soil. Sensible heat flux is the transfer of heat that you can sense (or measure with a thermometer) and is determined with a new technique called "surface renewal." Air temperature is measured four times per second with fine wire thermocouples. Sensible heat flux is calculated using turbulence statistics. The energy used for evapotranspiration of the water is calculated as the net radiation minus heat storage in the water and soil, minus the sensible heat transfer from the crop to the air.

The total seasonal water used (31.5 inches) was about 75 percent of what has been typically reported for California (42 inches). Crop coefficient values started at about 1.30 and decreased to about 1.09 when the canopy reached about 65 percent ground cover. During the main part of the season, these values remained at about 1.09 and then dropped to about 0.90 at the end of the season.

Thus preliminary indications are that current rice crop water use is less than what earlier projections indicate. Because these results are so different from earlier reports, the experiment is being repeated again in 2001 to confirm the findings.