Irrigation Water Management Considerations for Sandy Soils in Minnesota

Soil Water Monitoring Techniques

The status of the soil water for an irrigated crop needs monitoring regularly to assist the irrigation manager in making irrigation decisions.

Several soil water monitoring methods exist to assist in scheduling. The recommended method is a combination of an in-field monitoring and a daily soil water accounting system like the University of Minnesota's Extension Service "Irrigation Scheduling Checkbook Method" (FO-1322). If there isn't time to conduct a regular monitoring program, hire a crop consultant to monitor soil moisture. Brief descriptions of several available monitoring methods follow:

Soil Feel and Appearance. This involves soil sampling from several layers in the root zone and estimating the soil water deficit from soil feel and appearance. Table 5 gives a brief description of how some soil textures feel and appear for various soil moisture conditions. Take samples with a probe or shovel every 6 inches and add individual deficit estimates to determine the total soil water deficit. This method is fairly accurate but requires some field experience to learn the art of estimating consistently.

Soil Water Sensors. Sensors measure such items as soil tension or electrical resistance when placed in the soil profile. Several types of sensors exist. Laboratory developed charts like table 6 convert readings from sensors to soil water deficit values.

Soil tension or suction indicates the energy required by plant roots to extract water from soil particles. As soil water is removed its soil tension increases. Tension relates directly to soil water content. Soil tension is expressed in centibars or bars of atmospheric pressure.

Some sensors are portable but those field-placed for the season give best results, allowing soil water measurement at the same location throughout the season. Sensors are typically placed in pairs at one third and two thirds depth of the crop root zone and at two or more locations in the field. The most common sensors are discussed in the following.

• Tensiometer sensors are made from a porous ceramic tip sealed to the base of a water-filled plastic tube, sealed at the top with a removable air tight cap. A vacuum gauge connected to the tube measures the soil tension. Tensiometers work best for sandy soils because the vacuum gauge is only effective up to 80 centibars—equivalent to 50 to 70 percent soil water depletion for these soils. tensiometers require more preparation time and maintenance than electrical sensors.
  
  
• Electrical resistance sensors indirectly estimate soil tension by measuring the electrical resistance between two wire grids embedded in a block of gypsum, plaster, or a special material which maintains its moisture content in equilibrium with adjacent soil. The electrical resistance within the block varies with soil water content. A manufacturer's calibration curve converts the reading to soil tension. Then table 6 is used to estimate a soil water deficit for the specific soil. Some sensor models are more sensitive in the 0-100 centibars tension range which is a benefit for sandy textured soils. Resistance blocks require little preparation before installation and require no maintenance during the season.
  

  Table 6. Soil water deficit in inches per foot of soil for various tensions
  	Soil Tension - Centibars
  Soil Texture	10	30	50	70	100	200	1500*
  Coarse sands	0	0.1	0.2	0.3	0.4	0.6	0.7
  Fine sands	0	0.3	0.4	0.6	0.7	0.9	1.0
  Loamy sands	0	0.4	0.5	0.8	0.9	1.1	1.4
  Sandy loams	0	0.5	0.7	0.9	1.0	1.3	1.7
  Coarse sands	0	0.1	0.2	0.3	0.4	0.6	0.7
  Loams	0	0.2	0.5	0.8	1.0	1.6	2.2
  *Soil deficit at 1500 cbs is eual to total avaiable soil water capacity.

• Heat dissipation sensors are similar to the electrical resistance sensor but employ the principle of heat dissipation to estimate water content within a porous ceramic block. The rate of heat dissipation in the block is directly related to soil water tension. Sensors can read from saturation to over 300 centibars. These sensors are more expensive because the manufacturer individually calibrates them.

Soil Water Accounting. This method estimates the current soil water deficit from daily inputs of rainfall, irrigation depths, and estimated daily crop water use (ET). The daily accounting process is computed on a balance sheet like in a checkbook. Field gauges measure rainfall and irrigation amounts in the field. Depending on what weather data are available, estimate daily crop water use (ET) values by ET tables or research based models.

Figure 3. A typical Crop Water Use Pattern for Corn in Central Minnesota

ET tables give estimates of daily crop water use (ET) for different growth stages (for example, days after emergence) based on average weather conditions for a given region. ET tables exist for several field crops (for example, corn, potatoes, soybeans) grown in central Minnesota. Minnesota tables estimate daily ET by week after emergence and the maximum daily air temperature. Look for them in the " Irrigation Scheduling Checkbook Method" publication referred to on page 5. Minnesota tables tested out quite accurate in most years but still recommended is bi-weekly field verification of the estimated soil water deficit.

ET models are research-based empirical equations that estimate the daily potential ET for full cover grass or alfalfa crop using specific weather measurements such as solar radiation, temperature, humidity, and wind. To estimate potential ET for other crops, a correction factor is applied called a crop coefficient, which varies by growth stage. These crop coefficients are research developed and are specific to both crops and geographic regions.

There are several ET models available, such as Penman, modified Penman, Jensen-Haise, etc. In Minnesota the modified Jensen-Haise ET model gives reasonable success when used with crop coefficients developed by North Dakota researchers (Stegman et al., 1977).

ET models are most effective when incorporated into a user friendly computer program that allows the user to modify the crop coefficients and input weather and soil data. Several private and public computer software programs are available (Wisconsin-Curwen and Massie, 1986; North Dakota-Stegman and Coe, 1984).

Water evaporation devices such as the U. S. Weather Bureau class A pan can estimate daily crop ET when appropriate research-based crop specific correction factors are available. Crop curves have been published for a few crops in Minnesota but limited research has been done in developing curves (Seeley and Spoden, 1982). Farmers in western states have used in-field evaporation devices such as a wash tub or modified atmometer, but Minnesota experience has not produced consistent results.

Other Methods. There are several other methods available for helping an operator monitor soil moisture in the field but most are either too expensive or lack sufficient calibration for Minnesota use. Some of these include:

Neutron probe measures the actual soil moisture at various depths with a radiation source. The operator must be licensed and receive special training to use this. The unit is expensive and requires a lot of time to make field readings.

Heat dissipation sensors measures the temperature of the plants' leaves. Research shows that the leaf canopy to air temperature difference for a given crop, coupled with several other weather factors, can be related to the soil moisture stress the plant is experiencing at measurement time. This device is generally packaged with several other sensors and a small computer that can be carried to the field. The unit should be used only during full sunshine ( 11 a.m. - 2 p.m. ) for accurate measurements. The system is working well where cloud free days predominate and sufficient research data are available.

This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9180.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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