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<br />W <br />~,t);ig <br />. ~ ",- <br />',. Qj <br /> <br />3. During pre-irrigation of annual crops and first irrigation of <br />perennial crops for stand establishment (e.g. alfalfa), many <br />farmers in the Grand Valley apply a lar-ge amount of \oIater to <br />"b I ack over" the so i I so that seeds have adequate mo i sture for <br />germination and establishment. The elimination of this <br />practice \oIould reduce deep percolation substantially (see <br />#5). HO\olever, an alternative cultural or management practice <br />\oIOU I d be needed to ensu,-e stand estab Ii shment.. <br /> <br />4. Excess pre-irrigation, or the first irri.gation, is the <br />biggest contributor to seasonal deep percolation lo.sses. <br />This is directly related to high infiltration rate during <br />these irrigations, \oIhich subsequently decreases after the <br />first ir,-i'gation. This is supported by the "advance time" <br />data obtained from some of the monitored sites. When <br />infiltration is high, it takes longer'for \oIater to reach <br />the end of the furro\ol. The "advance time" .is less during <br />later irrigations because of the decrease in infiltration rate <br />due to seal ing of the so i I surface. <br /> <br />.;....,,\ <br />~~.:~:~ <br /> <br />5. The 1991 data for su,-face irrigated monitored sites <br />indicate that, on the average, about 3370 of the seasonal deep <br />percolation losses occurred during the first irrigation event <br />and about 4470 by the end of second irrigation. For annual <br />crops (small grain and corn), 4170 of deep percolation <br />occurred during the first irrigation and about 5970 by the <br />completion of the second irrigation. <br /> <br />6. The 1991 data also sho\ol that surface irrigated sites <br />had slightl.y more deep percolation than annual crops. <br />Normally, the reverse is true. Deep percolation for perennial <br />crops remained f~irly uniform throughout the season. Among <br />annual crops, the deep percolation dec,-eased drastically from <br />the f i ,-st to the second i rr i gat i on event and then gradua II y <br />decreased through the rest of the irrigation season. <br /> <br />7. Several years of monitoring data f,-om improved sur-face and <br />sprinkler irrigated sites indicate that the overall ave'rage <br />deep percolation (surface+sprinkler) for the Grand Valley is <br />approximately 16 acre inches per acre. When surface and <br />sprinkler irrigated sites are considered separately, deep <br />percolation is about 18 inches and 10 inches respectively. <br />Deep percolation from surface sites could be reduced by as <br />much as 30 to 5070 \oIith use of surge irrigation techniques. In <br />1990 and 1991, comparison of surge \oIith conventional sites <br />sho\oled that surge could also reduce total \oIater application <br />and tail\olater runoff by about 3070. Additionally, application <br />efficiency could be increased by 1070 \oIith surge. <br /> <br />12 <br />