Irrigation Scheduling Part 2 - Determining Water Holding Capacity

Last week we discussed how to calculate almond tree water usage by using evapo-transpiration data provided by CIMIS. Now that tree water use can be determined, we have to determine the amount of water that can be applied to the soil to maximize irrigation efficiency.

It is important to understand a few terms when dealing and discussing soil water holding capacity.  Soil saturation is when all of the soil pores are filled with water - this occurs immediately after an irrigation and continues until gravity pulls the water through the pores of the soil (Figure 1). Once the soil has drained, usually 1-3 days after an irrigation/rain event, the soil is at field capacity. This water is held against gravity within small pores by the soil particles, and is the largest amount of water that the soil can hold between irrigations. In contrast, the permanent wilting point is when all the available water is gone. The amount of water between the field capacity and wilting point is termed allowable depletion - or the amount water available to the plant provided by irrigations. It is important to note that at field capacity not all of the pore space is filled with water (about 25% air space), and at permanent wilting point not all of the water is removed from the soil (it is stuck to the soil particles).

Figure 1: Differing terms regarding water holding capacity of soils.

Basically, when efficiently irrigating, water needs to be applied to refill the soil profile to field capacity but prevent the loss of water to deep percolation. The amount of water to refill the soil profile is related to the texture of the soil; coarser soils hold less water while fine, heavy soils hold more. This is due to the differing size of soil particles and pore space in these soils. In general, soils are about 50% solids, 50% pore space. The pore spaces hold varying proportions of water and air. Table 1 shows the approximate available water of various soils.

Table 1: Available water for various soil types.

Type of Soil	Range (in/ft)	Average (in/ft)
Coarse(S / LS)	0.6-1.00	0.75
Sandy (LS / SL / L)	1.00-1.50	1.25
Medium (L / SCL)	1.25-2.2	1.50
Fine (SiL / SiCL / CL / SIC)	1.7-2.4	2.00

As we can see in table 1, differing soils have differing abilities in holding water. Therefore, determining how much water can be applied during an irrigation depends on the soil within the orchard. Soil sampling at various depths can help determine the soil texture throughout the root zone. It is advisable to to have soil maps from the NRCS on hand to help identify known texture changes. Root zone depth is usually determined by soil being excavated by an auger/probe, texturing the soil, and checking for roots. Most orchards, if the ground was prepared properly pre-plant will have between three to four feet of root zone. The presence of a clay lense, hardpan, or gravel layer may reduce the root profile depth and water availability to the tree. Once a soil texture profile has been developed, the amount of water that the soil can hold can be easily calculated by using the above table. For example:

Soil Surface	Soil Texture	Depth in Feet	Available Water Holding Capacity (From table1)	Available water in each soil layer (in)
1"-12"	Sand	1	0.6	0.6
13"-24"	Loamy Sand	1	0.8	0.8
25-42"	Sandy Loam	1.5	1.0	1.5
			Total:	2.9

In the example above, we have 2.9 inches available for the tree to use. When scheduling to irrigate, we shouldn't wait till all 2.9 inches are used, but rather base the timing of the irrigation when roughly 50% of the available water is used. Letting the profile go completely dry will deplete all deep soil moisture, affecting tree performance later in the season. Using the assumption of 50% also provides a bit of a "fudge factor," helping to prevent a mistake from being made. So, using the example above, irrigation should be scheduled  when 1.4-1.5 inches of water have been used by tree evapo-transpiration. Please review last weeks entry to help calculate evapo-transpiration.

Knowing the amount to water that can be held within your soil is also helpful in determining how much water is needed to refill the entire rooting profile. In the soil profile used in the example above, irrigating more than 3.0 inches may lead to deep percolation. Deep percolation may be of benefit if  leaching of salts from the rooting profile is desired.

For more information and estimating soil moisture by the "feel" method, please check Kern County's Irrigation Handout. Please email if any question comes to mind.