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crops and stage of growth reduce the
<br />amount. If ETr is used, it roust be modi-
<br />fied to reflect the crop type and matu-
<br />rity. Figures I and 2 are graphs of crop
<br />coefficients for corn and grain sorghum.
<br />Using these graphs, the ETr can be
<br />modified to reflect a field's growth,
<br />Record the crop coefficient (Koo) into
<br />the appropriate colurnm Multiply ETr by
<br />the Kco to obtain ET for your crop.
<br />Step 2. Determine the amount of effec-
<br />tive ralrjall.
<br />The amount of rainfall that actually
<br />enters the root zone Is the effective
<br />rainfall. The best estimation of cffea
<br />live rainfall will be based on observa-
<br />tion of the intensity and duration of the
<br />rainfall event. High intensity rainfall
<br />events exceed the soil infiltration
<br />capacity and increase runoff poteorial.
<br />High intensity rainfall, coupled with
<br />tong duration, would indicate large
<br />runoff volumes and result in low effec-
<br />tive rainfall. Low intensity rainfalls arc
<br />desirable since they more closely
<br />match the soil infiltration rate, thus
<br />more rain is effective. Long duration
<br />rainfalls have increased runoff' poten-
<br />tial since soil intake capacity decreases
<br />as water content increases. Precipita-
<br />tion of less thart 0.23 inches is usually
<br />ignored, and large events may require,
<br />soil sampling to determine the soil
<br />water levels to record on the water bal.
<br />ance sheet. In this example, the only
<br />rainfall event noted in Table 2 is 0.67
<br />inches occurring on 7117,
<br />Step J. Update the soil water stators.
<br />The irrigation amount recorded on
<br />Table 2 is not irrigation. The not irriga-
<br />tion will be based on the application
<br />rate as influenced by the irrigation
<br />efficiency. Irrigation efficiency is
<br />influenced by set length, time, furrow
<br />condition, furrow sheam size, length of
<br />run, tailwater reuse, and other factors.
<br />The net irrigation will be the gross
<br />application amount multiplied by the
<br />application efficiency of the system.
<br />Periodic soil sampling and general
<br />experience with a field will help an
<br />irrigator estimate net irrigation effi-
<br />ciency. In this example net irrigation
<br />application was calculated in Step D
<br />and was determined to be 3.0 inches.
<br />The two columns of Table 2 labeled
<br />Location I and Location 2 represent
<br />soil water conditions of the first and
<br />last set. The soil water depletion for the
<br />day is found using this formuiat.
<br />Soil Water Depletion - previous day's
<br />soil water depletion + ht - Net
<br />Irrigation - Effective Rainfall
<br />The example in Table 2 begins on
<br />June 29 with existing soil water deple-
<br />tion at both location I and 2 of 1.16_
<br />The calculation for June 30 Is-
<br />6130 depletion - 1.16*0.07-0-0>
<br />1.23. Example calculation for July 4 is
<br />depletion is 1.81 + 030 - 3,0 - 0 -
<br />-0.89, which shows the net irrigation
<br />of 3 inches. The negative value indi-
<br />cates excess irrigation application that
<br />cannot be stored in the mot none.
<br />Record 0.00 depletion for the date,
<br />Note in the net irrigation column that
<br />the number of days to finish the entire
<br />irrigation is shown along with the net
<br />irrigation amount that is recorded when
<br />the location receives the water.
<br />Stop 4. Begirt irrigation when the
<br />allowable soil water deptetion occurs.
<br />Table 2 also illustrates the difficulty
<br />in timing of the first irrigation. This
<br />difficulty is primarily due to the uni-
<br />form soil water condition of the field at
<br />the beginning of the first irrigation,
<br />Ottcc the first irrigation is completed,
<br />the soil water across the field is stag-
<br />gerel and, unless enough rainfall is
<br />received to refill the soil profile, it will
<br />remain staggered throughout the
<br />remainder of the irrigation season.
<br />In the example, the average of the five
<br />previous days ET are used as the pre-
<br />dictor of starting the irrigation. The
<br />average RIT from June 24 to June 28
<br />(data not shown) was 0.13 inches per
<br />day, If, for txamplet, 10 days are
<br />required to apply a 3-inch net apptice-
<br />tion, the estimated soil water with-
<br />drawal would be 0.13 x 10- 1,3
<br />inches. In order to prevent soil water
<br />depletion below the allowable limit of
<br />3 inches, irrigation should begin before
<br />1.7 inches of depletion (3.0 minus t.3)
<br />occurs in the last set. The irrigation is
<br />started on July 4, the first day after
<br />location 2 bad a depletion greater than
<br />'T'able I. Probable Itange of Surface Irrigatlon Efficiency
<br />under Various Manageme,tt Procedures*
<br /> Continuous flow, no tailwater recovery 50 to 60
<br /> Continuous flow, tailwater minimized 60 to 70
<br /> Continuous flow, tailwater reuse 70 to 85
<br /> Surge flow, tailwater minimized 70 to 85
<br /> Surge flow, tailwater reuse 75 to 90
<br />•Latgth of con, furmw strum stye. son nrpc, slope, jet titnr, etc. greatly influence irrigation
<br />effeioncy. Tt- etru:i-Y ranger ate guidwim eaeimaies,
<br />fiv- I. Co- Crop Coemaent vr. Figure 1-Crain Sorghum Crop Coefneienr vs.
<br />Stags orcrowth Stage of Growth
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