FRESNO, Calif. - According to WATERIGHT, an online educational resource for irrigation water management developed by the Center for Irrigation Technology (CATI) at California State University, "irrigation scheduling" is a generic term applied to any technique/practice intended to aid a professional in determining when and how much to irrigate. WATERIGHT said there are several ways to categorize different irrigation techniques, but they can be defined as either "water budget" or "graphical/sensor-based" methods. The following information describes WATERIGHT’s defined water budget method for irrigation scheduling.
WATER BUDGET IRRIGATION SCHEDULING EXPLAINED. All water going into and out of the Effective Root Zone (ERZ) - the depth of soil where an irrigator wants to control soil moisture, which may or may not be the full depth of the plant's roots - can be measured in terms of a depth of water. Therefore, if the amounts of the different types of water going into and out of the root zone can be identified, simple addition and subtraction can indicate the net change in root zone water content.
This is the basis for water budget irrigation scheduling techniques (also known as "checkbook" irrigation scheduling). Because this method focuses on day-by-day accounting of the amounts of water coming into and going out of the ERZ, it is important to note the following definitions of water depth:
- All of the water going into and out of the ERZ can be measured in terms of a depth of water, usually inches. Rainfall is measured in inches, and irrigations are commonly measured as acre-inches/acre, or just inches.
- Deep percolation is water soaked into the ERZ in excess of the soil's field capacity. Since it is either excess irrigation or excess rainfall, it too is measured in inches.
- If water going out of the root zone (deep percolation) is measured in inches, then upflux from a high water table ought to be measured in inches also.
- Finally, crop evapotranspiration is calculated in terms of acre-inches/acre per day, and if we are looking at any one day, just inches.
As will be seen shortly, the water budget equation is very simple. However, you must decide whether to solve the equation in terms of the soil moisture depletion, in terms of the available water, or in terms of total water in the effective root zone. It is most common solve in terms of either total water or soil moisture depletion. This discussion will solve in terms of total water since it makes it easier to understand the equation.
Assume a starting point as the total water in the effective root zone at the start of the day - call this Waterstart.
The total water in the root zone at the end of the day (24 hours later) will be called Waterend.
Then, for the day:
- Crop evapotranspiration will be called ETc.
- Rainfall that infiltrates the soil will be called RAIN.
- Any irrigation that infiltrates the soil will be called IRR.
- Any deep percolation from excess infiltrated irrigation or rainfall will be called DEEP.
- Any change in total water in the effective root zone from underground water movement (possibly a high water table or water moving laterally in the ground) will be called FLUXnet.
- Thus, the water budget equation is:
-
Waterend = Waterstart + IRR + RAIN - ETc - DEEP + FLUXnet
Identifying the values and solving this equation is the basis for water budget irrigation scheduling.
Find a continuation of this water budget irrigation scheduling tutorial click here: www.wateright.org/site2/advisories/sched1.asp.
The above information was reprinted from the WATERIGHT Web site at www.wateright.org.