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Determination of lower limits for irrigation management using in situ assessments of apparent crop water uptake made with volumetric soil water content sensors


, : Determination of lower limits for irrigation management using in situ assessments of apparent crop water uptake made with volumetric soil water content sensors. Agricultural Water Management 92(1-2): 13-28

In situ approaches for determining lower limit (threshold, refill) values for irriation g management using continuously monitored data from volumetric soil water content (SWC) sensors were evaluated. Four indices were derived from SWC data: (i) apparent daily crop water up take p (ADCVTU), the reduction in SWC during daylight periods; (ii) daily soil water loss (DSWL) the reduction in SWC during 24 h periods; (iii) previous overnight redistribution (POR) overnight T reshold, drainage; and (iv) ADCWU normalised for reference evapotranspiration (ADCWU:ET) Indices. were calculated for 0-20 and 20-40 cm soil depths in four drying cycles applied to melon and to autumn and spring tomato crops. In each drying cycle, there were well-watered and decay curve with three phases: (1) of rapid decline when drainage occurred; (2) of a much slowe and (3) of e constantly very low values not influenced by climate. The actual commencement of crop water stress was indicated by the first statistically significant difference in midday 11'1,,f between unwatered and well-watered plants. A protocol was developed to identif co y mmencement of crop water stress using ADCWU or DSWL in un-watered plants. POR was used to identify cessation of internal drainage. Then, the first reduction in ADCWU or DSWL while climatic variables (solar radiation, VPD) indicated a constant or increasing atmospheric evaporative demand was considered an "indication" of crop water stress, the second such reduction as 11 confirmation". In the four cycles studied "indications" of c, rop water stress using both ADCV,rU and DSWL for 0-20 cm soil were I (twice), 2 and 6 d after commencement f o water stress. When there was not appreciable climatic variation during phase 2, ADCVvrU and DSVIL detected crop water stress within 1-2 d Of'I'l-T. Generally, climate -mediated fluctuations i n ADCWU and DSWL during phase 2 delayed identification of the commencement of t wa er stress. Norm alising AD CVTU forET, reduced effects ofclimatic fluctuation. Using a 25% relative reduction in ADCWU:ET as a threshold, this parameter was more sensitive than ADCV U or V DSWL using the proposed protocols for detecting commencement of water stress. "Breaking " points in the rate of reduction of SWC, and variations in stem diameter were also compared for detecting commencement of crop water stress. This study suggested that reductions in SWC in drying soil can be used to identify lower limits for irrigation management usin SWC g sensors, and that stable climatic conditions are required to optimise these approaches. C) 2007 Elsevier B.V. All rights reserved.

US$19.90

DOI: 10.1016/j.agwat.2007.04.009


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