Soil Moisture and Irrigation
Most soil moisture sensors provide measurements in the unit “water fraction by volume” (wfv or m3m-3) and is symbolized with the Greek letter theta (θ). Multiplying the water fraction by volume measurement by 100 will equal the volumetric percent of water in soil. For example, a water content of 0.20 wfv means that a 1 cubic meter soil sample contains 200 cubic centimeters of water, or 20% by volume. Full saturation (all the soil pore spaces filled with water) occurs typically between 0.35-0.55 wfv for mineral soil and is quite soil-dependent.
There are a number of other units used to measure soil moisture. They include % water by weight, % available (to a crop), inches of water to inches of soil, % of saturation, and tension (or pressure). The conversion between units can also be highly soil-dependent.
Because the bulk density of soil is so highly variable, soil moisture is most meaningful as a water fraction by volume or volumetric percent. If weight percent were used, it would represent a different amount of water from one soil texture to the next and it would be very difficult to make comparisons.
Soil Moisture Measurement Considerations for Irrigation
Soil moisture values are particularly important for irrigation optimization and the health of a crop. There are two different approaches for determining an irrigation schedule from soil moisture data: the fill point method and the mass balance method.
Other common irrigation scheduling methods that do not include soil moisture sensors use evapotranspiration (ET). ET is the rate of water leaving the soil by the combination of direct evaporation of water out of the soil and the amount of water being transpired by the crop. ET can be thought of as negative precipitation. ET is determined from calculations based on meteorological conditions such as air temperature, solar radiation and wind.
The most common ET irrigation scheduling determination is called the Penman-Monteith Method, published in FAO-56 1998 Food and Agriculture Organization of the UN. The FAO 56 method is also a mass balance approach where the amount of water that is leaving the soil can be determined and matched by the irrigation schedule. In practice, due to the high importance of the success of the crop, ET methods in combination with soil sensor data can be used by irrigators to best manage irrigation.
Fill Point Irrigation Scheduling
The fill point method is qualitative in that the irrigator looks at changes in soil moisture. With experience and knowledge of the crop, an irrigation schedule can be developed to fill the soil back up to a fill point. The fill point is an optimal soil moisture value that is related to the soil’s field capacity. The fill point for a particular sensor is determined by looking at soil moisture data containing a number of irrigation events. This can be an effective and simple way to optimize irrigation. Because it is qualitative, accuracy of the soil moisture sensor is less important because the fill point is determined by looking at changes in soil moisture and not the actual soil moisture itself. This in some ways can be more efficient because lower cost soil moisture sensors can be used without calibration. While the fill point method can be easy to implement and is widely used for many crops, the mass balance method however can better optimize the irrigation, provide better control of salinity build up, and minimize the negative impacts of over-irrigation.
Mass Balance Irrigation Scheduling
The mass balance method (sometimes called scientific irrigation scheduling) is an irrigation schedule determined by calculating how much water is needed based on accurate soil moisture readings and the soil properties. Equations ,  and  (below) can help to determine how much water to apply. The following are terms commonly used in soil hydrology:
Soil saturation (θSAT) refers to the situation where all the soil pores are filled with water. This occurs below the water table and in the unsaturated zone above the water table after a heavy rain or irrigation event. After the rain event, the soil moisture (above the water table) will decrease from saturation to field capacity.
Field Capacity (θFC) refers to the amount of water left behind in soil after gravity drains saturated soil. Field capacity is an important hydrological parameter for soil because it can help determine the flow direction. Soil moisture values above field capacity will drain downward recharging the aquifer/water table. Also, if the soil moisture content is over field capacity, surface runoff and erosion can occur. If the soil moisture is below field capacity, the water will stay suspended in between the soil particles from capillary forces. The water will basically only move upward at this point from evaporation or evapotranspiration.
Permanent Wilting Point (θPWP) refers to the amount of water in soil that is unavailable to the plant.
Allowable Depletion (θAD) depletion represents the amount of soil moisture that can be removed by the crop from the soil before the crop begins to stress.
Lower soil moisture Limit (θLL) is the soil moisture value below which the crop will become stressed because it will have insufficient water. When the lower limit is reached, it is time to irrigate.
Maximum Allowable Depletion (MAD) is the fraction of the available water that is 100% available to the crop. MAD can depend on soil or crop type.
Available Water Capacity (θAWC) is the amount of water in the soil that is available to the plant.
The lower soil moisture limit is a very important value because dropping to or below this value will affect the health of the crops. The equations below show how to calculate the lower soil moisture limit and the soil moisture target for irrigation optimization.
θAD = (θFC – θPWP ) x MAD [ 1 ]
θAWC = θFC – θPWP [ 2 ]
θLL = θFC – θAD [ 3 ]
|Crop||Maximum Allowable Depletion (MAD)||Effective Root Depth (Inches)|
The following graph can be used to help determine the soil moisture targets based on soil texture. Soil texture is determined by the percentages of sand, silt, and clay using the soil textural triangle. The actual MAD, field capacity and permanent wilting point varies with region, soil morphologies, and the crop. Note that these are general trends.
The soil is a silt, the MAD is 50%, and the soil moisture is 20% throughout the root zone which is down to 24 cm. The sprinkler is 75% efficient. How much water should be applied?
Answer: From the table above, the MAD = 0.5. From the graph (or a soil survey) the permanent wilting point (θPWP) = 16% and the field capacity (θFC) is 32%. Using the three equations above, the optimal soil moisture is 24% to 32%. θFC – θ = 32% – 16% = 16%. Therefore, the soil needs to be irrigated to increase the soil moisture by 16% down to 24 cm, 16% X 24 cm = 3.8 cm of water applied.
If the sprinkler is 75% efficient then 3.8 cm/0.75 = 5.12 cm of water should be applied. Note the rate of water coming out of the sprinkler should not exceed the infiltration rate of the soil and the run time of the sprinklers would depend on the specification of the sprinkler.