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Water Budget
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Water budgeting reflects a balance between the inputs and outputs of water to and from the plant root zone. The method is similar to balancing a checkbook. Water budgeting inputs include precipitation, irrigation, dew, and capillary rise from ground water. The outputs include evapotranspiration, runoff, and deep percolation.
Evapotranspiration is the loss of water to the atmosphere by the combined processes of evaporation and transpiration. Transpiration is water transfer to the air through plant tissues.
Reference Evapotranspiration (ETo/ETr) is a term used to describe the evapotranspiration rate from a known surface, such as grass or alfalfa. ETo is expressed in either inches or millimeters. The ETo for an average year is referred to as normal year ETo.
Crop Coefficients (Kc) are used with ETo to estimate actual evapotranspiration (ETc) for a specific crop.
CIMIS provides ETo estimates by measuring weather parameters such as wind speed, air temperature, solar radiation, etc. at its weather station sites, and then calculates ETo values using the modified Penman and the Penman-Monteith equations.
CIMIS also provides measured values of precipitation at most of its weather stations. The user must determine the remaining soil-water budget components needed to use this method. Runoff and deep percolation can be estimated based on local factors such as soil properties and slope. Dew formation and capillary rise of ground water can also be estimated. However, estimation of the latter two parameters is a bit involved. Fortunately, these values are relatively small (depending on the specific situation) and can be ignored for most practical purposes. There are several tools and resources that an irrigator can utilize in order to estimate these parameters and schedule his/her irrigation needs.
Many resources are available to help you create an irrigation schedule that meets your specific needs. Visit our technical references to get a listing of publications that you can download or request. Check this site often as new resources and information will be updated periodically.
Consultants are available to help you create your specific irrigation schedule. Consultants are hired to advise the grower or manager on when to irrigate and how much water to apply.
There is also a list of DWR staff that can point you in the right direction.
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