Tehama County - Weekly Soil Moisture Loss Reports - Irrigation and Water Resources

Weekly Soil Moisture Loss Reports

Source of Weekly Soil Moisture Loss Reports

The Northern District of the California Department of Water Resources and the University of California Cooperative Extension in Red Bluff work together to provide a weekly report of soil moisture loss (Evapotranspiration or ET) estimates for some major crops grown in the northern Sacramento Valley during the irrigation season (April through October). Weekly reports for the 2009 season are posted each Friday and are listed below for your viewing or downloading. Archived reports for the 2005 and 2008 seasons are also available at the end of this page.

2009 Soil Moisture Reports

2008 Soil Moisture Loss Reports

2007 Soil Moisture Loss Reports

Sample Report: Each report provides weekly estimates of soil moisture loss for a specific week and a running total of the soil moisture loss for the season. A sample report for 2007 is provided below along with an explanation of how to use the information.

View larger version of this example chart.

How can these weekly soil moisture loss reports help decision-making? They can help judge how well rainfall balances the soil moisture loss, when to begin supplementing rainfall with irrigation, and how long to operate your irrigation system to replace the soil moisture loss. The soil moisture loss estimates are most conveniently used with drip, micro-sprinkler, or sprinkler irrigation where irrigation water is applied more precisely and the hourly water application rate can be determined in inches of water per hour.

Example of how to use this report in decision-making: The table above shows accumulated seasonal water use for almonds grown west of the Sacramento River from March 1 through March 29, 2007 was 2.36 inches while accumulated rainfall since February 23, 2007 was 0.54 inches. Rainfall has fallen short of the soil moisture loss from almonds during this period, suggesting a soil moisture deficit is developing and that modest levels of irrigation with micro sprinkler or drip may be appropriate. But, to be more confident some type of field verification to check soil moisture levels or crop stress levels is encouraged. In the case of almonds, which have had a full canopy for a few weeks, 0.74 inches of soil moisture loss was estimated from March 23 to March 29, while rainfall was 0.00 inches. If a micro sprinkler system were in use with an hourly water application rate of 0.06 inches per hour, similar to the one described below for an almond orchard, it would require about 12 hours of irrigation to replenish the past seven days of soil moisture loss from almond.

What if the hourly water application rate is unknown? Determining the hourly water application rate of a drip or micro sprinkler system requires knowing the number of drip or micro sprinkler emitters per acre, the specific emission rate of the drip or micro sprinkler emitter, and remembering that 1.0 inch of water covering an acre of land (acre-inch) equals 27,154 gallons. Refer to the example. Contact the Tehama County Farm Advisors office for assistance with impact sprinkler and flood or furrow irrigation systems.

How is soil moisture loss (ET) estimated? The estimates are based upon hourly measurements of sunlight, temperature, humidity, and wind obtained from weather stations located near Gerber (westside) and Durham (eastside). Estimates are for specific crops to adjust for different leaf-out and planting dates and, in the case of orchard crops, for orchard floors with resident vegetation that are managed using strip applications of herbicides and mowing.

How reliable are the ET estimates? The weather stations have a long history of operation and are maintained regularly. They are automated so that data is recorded hourly. Estimates are for healthy crops where soil moisture is plentiful and the management goal is to avoid crop stress. In the case of fruit and nut crops, the estimates are for mature, productive orchards. They will overestimate ET for unhealthy crops, young non-bearing orchards with smaller canopies, or for periods of crop development when water stress might be beneficial. They may underestimate where cover crops are growing vigorously in orchard middles.

Since these are regional estimates they should be supported with field monitoring and crop observation. Information is available on techniques for measuring crop stress in orchard crops with a pressure chamber and manually operated and automated methods of monitoring soil moisture in either orchard or agronomic crops.