Kanemasu, E. T.

Powers, William L.

Steiner, Jean

K-State Research Exchange

' JANUARY 1980 Using a Programmable Calculator To Schedule lrrigation1 Jean Steiner, E. T. Kanemasu, and W. L. Powers2 Water and energy resources are becoming increasingly scarce and valuable. To preserve these resources and to hold the costs of production down, improved irrigation management techniques are being examined. One of the ways to improve irrigation management is to understand water use by crops throughout the season. Evapotranspiration (ET) is the amount of water lost through evaporation f rom the soil surface and transpiration from the plants. On a given day, ET from a field depend s pri-mari ly on the type of crop, the amount of crop cover, surface soil moisture, and climatological conditions. ET can be estimated mathematically, using f ield and climatic data. ET estimates have been made at Kansas State Univer-sity for several years, but the information has been used primarily for research purposes, because most people in the state don't have access to computer faci lities. We have simplified the K-State ET moclel so it can be run on a programmable calculator and so the d ata necessary to run the model are re lative ly easy to collect. 1. Contribution No. 80-2 13-S. Agronomy Department, Eva potranspiration Labo ratory, Kansas Agricultura I Ex-periment Station, Kansas State Unive rsity, Manhattan, KS 66506. 2. Research assistant, research micrometeorologist, and soi I physicist, De partment of Agronomy, Kansas State University. AGRICULTURAL EXPERIMENT STATION Kansas State University, Manhattan Floyd W. Smith, Director For a full cover crop, ET depends primarily on the amount of energy available to evaporate water, and therefore depends on solar radiation and air temperature. At less than full cover, ET depends primarily on the crop cover and the surface soil moisture. To use the ET model for a field, one must know the soil type, the water holding capacity of the soil, and the initial moisture in the soil profile, as well as daily maxi-mum and minimum temperature, solar radiation, leaf-area index, rainfall, and irrigation. The model then estimates evaporation and transpiration and calculates a soil mois-ture balance for the day. Solar radiation and temperature measurements are available from the National Weather Service and from seve ral Agricultural Experiment Stations. leaf-area index is the ratio of green leaf surface area to soil surface area and is determined by measuring the leaf area of representative plants and multiplying by the number of plants in a given soil area (plant population). The best results are obtained by measuring the leaf-area index in each field, but since a crop follows a specific pattern of growth and development, leaf-area index could be estimated from generalized curves. a: w w w ...J ...J uJ ...J ..J-(f) T 60 100 .3 .2 w w TRANSPIRATION DATE Figure 1. Average daily evapotranspiration, soil mois-ture depletion (of a Ulysses silt loam with 12 .6 inches of m3ximum available water), and growth stages of corn. After the da ily ET is the soil moisture bal-ance is updated by adding rainfall or irrigation and sub-tracting ET, runoff and deep percolation. An irrigator who knows the soi l moisture status of his fields and the rate a crop uses water can make better management de-cisions. An example of ET and soil moisture balance information is presented in Figure 1 . Soil Moisture Balance The soil moisture balance is calculated daily by the model. A weekly soil moisture balance from July 16 through August 13 is calculated in a similar way for the same field as shown below. Soil Moisture Information To maintain a soil-moisture balance, you must f irst know how much water is in the soil profile and how much of that water is available for uptake by the plants. The bes~ way to determine the in itial soil moisture is to take a soil sample with a probe and weigh the samples while they are moist, then after they dry. For example, (moist weight - dry weight)/ dry weight = 22.6% {.226) by weight. Assuming a bulk density of 1.3, the moisture percentage by volume is 22.6 X 1.3 or 29.4%. Total moisture in a 5-foot profile is then ,294 X 60 inches = 17.64 inches. To dete rmine maximum available water (AW ), you max must know field capacity (FC) and permanent wilting point (PWP). Both are available from the Soil Conserva-· lion Service or from county extension agents. For our sample calculations, we consider FC = 36.8% and PWP = 15.6%. To find the amount of water the soil can hold easily available to plants, you subtract PWP from FC. In this case, AW = FC - PWP = 36.8 - 15.8 max = 21.0%, which is the water than can be managed for crop production. The maximum available water is .210 X 60" = 12.6 inches. Sample Calculations: Corn, Grant County, Kansas, 1978 Ulysses Silt Loam - Maximum available water = 12.6" July 17..July 23 Ini tial soil moisture Rainfall Average daily ET .29" X July 24-July 30 Initial soil moisture Irrigation Average daily ET .3 1" X Total soil moisture, inches 17.64 + .68 - 2.03 7 days 7 days 16.29 + 2.01 - 2.17 16.13 Avail. soil % mois .. de pie• 1ion ture, inches 35 8.27 45 6.92 47 6.76 July 31-August 6 Initial soil moisture Rainfall Average daily ET .20" X August 7-August 13 Initial soil moisture Rainfall Runoff Irrigation Average dai ly ET .27" Sche~uling Irrigation X 16.13 + .17 1.47 7 days 14.83 7 days 14.83 + 1.03 .01 + 2.01 1.89 15.97 56 5.46 47 6.60 The irrigator in this example scheduled his water a p-plications to allow a gradual depletion of the stored soil moisture throughout the season, without allowing it to drop low until the season ended. A late July irriga-tion assured adequate soil moisture during the tasse l and silking stages, which is criticaL He did not completely refill the soil moisture profile when he irrigated, so rain-fall could be beneficially used by the crop, and he grad-ually allowed the soil moisture reserve to . be depleted as the crop matured. An ET model helps an irrigator who wants to limit the water he applies to his crops. It allows him to monitor the deple tion of water from his soil profile so that he knows when additional water must be added. It also gives him an idea of the rate water is used, so he can decide whethe r he wants to replace all or part of the WJter that the crop depletes from the soil. Agricultural Experiment Station Kansas State University Manhattan 66506 Keeping Up With Research 45 January 1980 Publications and public meetings by the Kansas Agricultural Experiment Station are a.,.ailable and open to the public regardless of race, color, national origin, sex, or religion. 1-80- 3M 

Using a programmable calculator to schedule irrigation

http://krex.k-state.edu/dspace/bitstream/2097/16638/1/SRL45.pdf

Using a programmable calculator to schedule irrigation

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