2011-08-04_ENFORCEMENT - C1981008 (4)

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Deficit Irrigation of Alfalfa for Water- Savings in the Great Plains and Intermountain West: A Review and Analysis of the Literature R. Bradley Lindenmayer, Neil C. Hansen,* Joe Brummer, and James G. Pritchett ABSTRACT Diversions of water from irrigated agriculture are occurring in the western United States to address increasing municipal and industrial demands. Deficit irrigation of alfalfa (Medicago sativa L.) could be a source of water without complete dry-up of irri- gated fields. Water saving potential from alfalfa is high because it is a high water -use crop produced on 12% of the irrigated land in the United States. The objectives of this paper are to review alfalfa plant -water relations in the Great Plains and Inter- mountain West, to understand potential water savings through deficit irrigation, and to indentify management practices that maximize water -use efficiency (WUE). Alfalfa biomass yield exhibits a linear relationship to evapotranspiration (ET) with the slope of a regionally aggregated water production function of 0.16 Mg ha -1 cm - I. Relative ET declines 30% faster than relative biomass yield under deficit irrigation or dryland management. Because early season harvests have greater WUE, combining full irrigation in spring with no irrigation during Icss efficient water -use growth periods may be more effective in saving water than season -long deficit irrigation. Management practices that can influence WUE under deficit irrigation include stand age, growth stage at harvest, and alfalfa variety. A potential complication with controlled deficit irrigation of alfalfa is an uncertain contribu- tion to ET from a water table. As alfalfa roots develop over time, a significant percentage of total ET can come from water tables shallower than 200 cm and the percentage increases as availability ofwater from precipitation or irrigation declines. I N WATER-SCARCE ENVIRONMENTS, demand for water exceeds the available supply, creating competition among users for this limited resource. Population growth in the western United States drives water scarcity and the problem is magnified by drought conditions and declining groundwater levels (McGuire, 2004; Dugan and Sharpe, 1996). As an example, the Statewide Water Supply Initiative compiled by the Colorado Water Conser- vation Board (2004a, 2004b) projects that Colorado's population will grow by 65% within the next 25 yr resulting in a likely dry-up of 170,000 ha of irrigated farmland to meet the urban demand for water. Similar water reallocation from agriculture to municipali- ties has the potential to dry up irrigated agriculture throughout much of the western United States. Solutions that meet water demands of growingpopulations and industry without loss of irrigated land are needed to preserve food production and regional economies that depend on irrigated agriculture. Because alfalfa is a high water -use crop (Stanberry, 1955; Schneekloth and Andales, 2009) grown on 12% of the irrigated land in the United States (National Agricultural Statistics Ser- vice, 2007), controlled deficit irrigation of alfalfa has been pro- posed as a source of water transfers from agriculture (Putnam R.B. Lindenmayer, N.C. Hansen, and J. Brummer, Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523 -1170; J.G. Pritchett, Dep. of Agricultural and Resource Economics, Colorado State Univ., Fort Collins, CO 80523. Received 17 May 2010. "Corresponding author (neil. hansen r@r colostate.edu). Published in Agron. J. 103:45 -50 (2010) Published online 1 Nov 2011 d o i:10.2134/ ag r o n j 2010.0224 Copyright © 2010 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. et al., 2005). Controlled deficit irrigation is an approach that supplies water at a rate below the full crop water requirement and usually results in lower biomass yields than full irrigation (Carter and Sheaffer, 1983; Grimes et al., 1992; Undersander 1987). Established alfalfa may be adapted to deficit irrigation with drought avoidance mechanisms such as deep rooting (Peterson, 1972) and drought induced dormancy (Peterson, 1972; Smith, 1962; Robinson and Massengale, 1968). The objectives of this paper were to review alfalfa plant -water relations in the Great Plains and Intermountain West regions of the United Stares in an effort to identify practices to maximize WUE under deficit irrigation. Irrigated alfalfa studies were com- pared to evaluate differences in ET, biomass yield, WUE, and the potential for water savings from controlled deficit irrigation. Evapotranspiration, Biomass Yield, and Water - Use Efficiency under Variable Irrigation Studies conducted in the Great Plains and Intermountain West of the United States were compared to evaluate alfalfa ET and biomass yield under variable irrigation (Daigger et al., 1970; Bauder et al., 1978; Retta and Hanks, 1980; Sammis, 1981; Carter and Sheaffer, 1983; Undersander, 1987; Wright, 1988; Bogler and Matches, 1990; Smeal et al., 1991). Only literature sources that had a complete accounting of water use from all sources, including irrigation, precipitation, and soil water, were included in this comparison. The majority of these studies used a water balance approach to determine ET. Studies that did not account for soil water changes or that may have been confounded by uncontrolled water use from a water table were excluded. Our analysis revealed that growing season ET averaged 91 cm over full irrigation treatments, 80 cm for deficit Abbreviations: ET, evapotranspiration; WUE, water -use efficiency; Gc, cumulative growing-degree-days; Sd, average daily solar irradiance Agronomy Journal • Volume 103, Issue I • 2011 45