Irrigation Chapter 9 - Soil Water Balance

In previous chapters we learned the basics of measuring soil water and determining plant water requirements­ and the efficiency of various irrigation systems. In this chapter we will learn the importance of knowing the soil water balance — the amount of water held in the soil at any given time. We also will learn the inputs needed to determine soil water balance and how soil water balance is used for irrigation management. The information learned in this chapter will allow you to determine the soil water balance at any time during the growing season.

Most irrigation systems are operated to manage soil water content for optimum crop growth. Knowing the soil water balance is essential for good irrigation management. Soil water balance refers to the amount of water held in the soil and is similar to a checkbook balance. Because soil can hold a limited amount of water, knowing the soil water balance reduces­ the risk of applying too much water resulting in deep percolation or runoff. It also assures that irrigation can occur in a timely fashion to avoid crop stress. The process of applying the right amount of water at the right time is irrigation scheduling. Without knowing the soil water balance, irrigation scheduling is not possible. Irrigation scheduling will be discussed in Chapter 10.

Author: Dean Yonts, University of Nebraska Lincoln Extension Biological Systems Engineer, Panhandle Research­ and Extension Center, Scottsbluff, NE.

Water storage capacity in the root zone is determined by soil texture and plant growth stage. Table 9.1 gives the available water capacity soil texture classifications. These values represent the amount of water in each foot of soil that is available to the plant. For most crops 50 percent of the available water capacity­ can be used before plant stress begins. Late in the growing season this value can increase due to full root development and a reduction in evapotranspiration. Table 9.2 provides the rooting depth for crops at various growth stages. By using the information from these two tables, total available water can be determined.

¹Lower minimum water balances may be desirable during some crop growth stages in water short areas or if pumping cost are high. A minimum water balance of 40% is generally recommended for late season water management.²Inches of water per foot of active root zone

¹Restrictive layers (hardpans, compacted layers, or layers of coarse material) can reduce the rooting depth.

For example, let’s determine the available water capacity of a sandy loam soil. Corn is being grown and it is in the early tassel stage of growth. In Table 9.1 the available water capacity is given as 1.4 in/ft. From Table 9.2, corn at early tassel has an approximate rooting depth of 2.5 feet. Multiply the rooting depth by available water capacity to determine the total water holding capacity (2.5 x 1.4 = 3.5 inches).

Total available water capacity, discussed in Chapter­ 3, is the quantity of water held in the root zone when the soil water content is at field capacity. Any additional water added to the soil will result in surface runoff or deep percolation of water below the root zone. Total available water capacity is the maximum amount of water that can be held in the soil and be available to the plant.

In Table 9.1 the minimum balance, as defined in Chapter 3, is given for different crops based on different soil textures. This value gives the minimum amount of water held in the soil before crop stress begins­, approximately 50 percent of the available water­ capacity. The goal of irrigation management is to maintain a soil water balance between field capacity and a minimum balance . As a result­ water can be applied before plant stress occurs and without over filling the root zone.

To begin the soil water balance, first determine the soil water content. If you are sure excess rainfall or irrigation has occurred, soil water content can be assumed to be at field capacity. Using Tables 9.1 and 9.2, you can then estimate the beginning soil water content based on soil texture and rooting depth. The preferred method is to measure the current soil water­ content using one of the techniques described­ in Chapter 4.

The next step is to measure effective rainfall. Place a rain gauge near the site and away from any obstructions­. Rainfall varies even over short distances, so more than one gauge may be needed. A rule of thumb is to use a minimum of one rain gauge for each quarter section of land. A better estimate can be achieved by using one rain gauge per 40 acres. The rain gauge shows the gross amount of rainfall, not the effective rainfall. Several factors influence effective rainfall, including­ field slope, soil texture, soil structure, plant cover, crop residue, storm intensity and storm duration. During rainfall, water either infiltrates into the soil or runs off the field. The water that infiltrates the soil is considered effective as long as there are no deep percolation losses.

You can use the rain gauge measurement, but you will need to visually estimate how much water infiltrates the soil by observing the rainfall.

Just as not all rainfall is effective, neither is all irrigation­ water. Some water is lost to evaporation and runoff which decreases the efficiency of an irrigation system. Irrigation system efficiencies were discussed in Chapter 8. Gross application, the total­ amount of water applied during irrigation, is multiplied­ by the system efficiency to obtain the net irrigation amount.

The amount of water used by the crop since the last soil water balance update is the final piece of information­ needed. In Chapter 6we discussed plant water requirements. Water use can be estimated using­ this information or can be more accurately estimated using current weather data that provides crop water use values on the web.  Another method is to use an ET gage.  The ET gage simulates the potential for a crop to use water.  This information in combination with specific crop coefficients can provide crop water use throughout the growing season.

A final method of estimating crop water use is based on stage of plant growth. Table 9.3 gives various crops grown in Nebraska. A water use rate (inches per day) is given for different growth stages. These values are based on long-term weather records and reflect an average water use rate. Actual water use can be higher or lower depending on  current weather conditions. Crop water use by growth stage is best used for system design or when long-term crop water needs are required. When scheduling irrigations, daily or weekly water use rates measured or calculated from current  weather conditions are more accurate.

¹Alfalfa water use rates should be multiplied by 0.50 during the first ten days following cutting and by 0.75 for the 10^th to 20^th day following cutting.²Taken from: Wright, J.L., J.C. Sturk. In Press. Irrigation of Potatoes. In American Society of Agronomy Monograph:  “Irrigation of Agricultural Crops” Section VII, Chapter 29.

Calculating the soil water balance is much like balancing a checkbook. You add any deposits since the last update and subtract any withdrawals. Effective irrigation and rain are considered deposits to the soil “bank” and added to the beginning soil water balance. Withdrawals from the soil “bank” occur through crop water use. The beginning soil water balance plus any effective rain or irrigation minus the crop water use gives the current soil water balance. This assumes there is no soil water lost or added from below the active root zone. To update the soil water balance weekly, use the soil water balance last calculated, add effective rainfall and irrigation, subtract crop water use and obtain a new soil water balance.

Soil water content, effective rainfall, effective irrigation and crop water use are all estimated values. Once a soil water balance is calculated, it should be updated periodically by measuring  soil water content (See Chapter 4).

Following is an example of maintaining a soil water balance during a three-week period.

     Corn at silking stage Table 9.2

     Sandy loam soil Table 9

     Beginning soil water content is at field capacity

     Week 1

               Effective rainfall = 0.0 in

.               Effective irrigation = 0.0 in

.               Crop water use = 1.8 in

.      Week 2

                 Effective rainfall = 0.5 in

.                Effective irrigation = 1.5 in

.                Crop water use = 1.9 in

.      Week 3

                 Effective rainfall = 1.0 in

.                Effective irrigation = 2.0 in

.                Crop water = 2.1 in.

Beginning soil water content = 3 ft. (rooting profile, Table 9.2) x 1.4 in./ft. (available water capacity, Table 9.1) = 4.2 in.

Minimum balance = 3 ft. (rooting profile) x 0.7 in./ft. (minimum water balance, Table 9.1) = 2.1 in.

Goal: Maintain soil water balance between 4.2 in. and 2.1 in.

Week 1

     Beginning soil water balance     4.2 in

.      Effective rainfall                       + 0.0 in

.      Effective irrigation                    + 0.0 in

.      Crop water use                        – 1.8 in.

      New soil water balance               2.4 in.

Week 2

     Beginning soil water balance     2.4 in.

     Effective rainfall                       + 0.5 in.

     Effective irrigation                    + 1.5 in

.    Crop water use                        – 1.9 in.

     New soil water balance              2.5 in

Week 3

     Beginning soil water balance      2.5 in

.    Effective rainfall                       + 1.0 in.

     Effective irrigation                    + 2.0 in.

     Crop water use                        – 2.1 in.

     New soil water balance               3.4 in.