畦田自然要素时空变异性及其对灌水质量的影响

Spatial-temporal variability of natural factors in border fields and its effect on irrigation performance

土壤入渗参数、田面糙率、坡度等自然要素是畦灌方案设计的基础参数,其变异性是导致灌水质量远低于预期的重要原因。为揭示畦田自然要素时空变异特征及其对灌水质量的影响,该研究在11组畦田上开展了4 a畦灌观测试验,通过实测数据全面研究了灌前自然要素的畦内空间变异性、畦间空间变异性和年际时间变异性,并结合WinSRFR地面灌溉模型,模拟分析了自然要素时空变异性对畦灌水流运动过程及灌水质量的影响。结果表明:对畦灌灌水质量影响最大的3个自然要素依次是入渗系数、入渗指数和糙率,三者空间变异系数平均值分别为11.00%、4.05%和7.94%,时间变异系数平均值是分别为26.87%、7.73%和21.86%。畦田自然要素变差导致灌水质量整体下降,自然要素的时间变异性对灌水质量的影响大于其空间变异性的影响。入渗参数和糙率的变异性对畦田后半段的水流消退过程影响较大,忽视其年际时间变异性会加大畦尾积水风险。该研究结果可为畦灌方案设计提供指导,为精细地面灌溉发展提供科学依据。

Natural factors have been the most important parameters in the border irrigation design,such as infiltration coefficient(k),infiltration index(α),roughness(n),and slope(s0).The irrigation performance is significantly influenced by the variability of these factors.In this study,a four-year border irrigation experiment was conducted on 11 sets of border fields,in order to reveal the spatial-temporal variability of natural factors and their impact on irrigation performance.The measured data was collected to analyze the spatial variation in the natural factors between and within the borders,as well as the interannual temporal variability of natural factors during the irrigation period.WinSRFR model was combined to simulate the impact of spatial-temporal variation in the natural factors on the flow process and irrigation performance of border irrigation.The results showed that:1)The average spatial variation coefficients of k,α,s0,and n of the borders were 11.00%,4.05%,10.28%,and 7.94%,respectively,while the average temporal variation coefficients were 26.87%,7.73%,4.70%,and 21.86%,respectively.Three natural factors presented the greatest impact on the irrigation performance:k,α,and n,all of which were greater temporal variability than spatial one.2)Natural factors(k,α,and n)shared a greater impact on the flow recession process than those on the flow advance process.The natural factors presented a small impact on the flow recession time in the first half of the border,but there was a great impact on the second half of the border during the recession.Particularly,a significant impact was found at the end of the border.Therefore,the spatial-temporal variability of natural factors(k,α,and n)increased the risk of waterlogging at the end of the border.3)The average spatial variation coefficients of k,α,and s0 within the border were 15.98%,7.83%,and 45.73%,respectively.There was a greater spatial variability of natural factors within the border,compared with the between borders.The variability of natural factors within the border also reduced the irrigation performance.Therefore,high irrigation performance was highly required to consider the natural factors within the border in the design of precision surface irrigation.4)The variation of natural factors in border fields resulted in low irrigation performance,indicating the greater impact of temporal variability on irrigation performance.Therefore,it was recommended to select a typical border from farmland,in order to measure the natural factors before irrigation.And then the irrigation inflow rate and distance-based cut-off were taken as the design variables,and irrigation performance as the optimization objective.The optimal combination of irrigation inflow rate and distance-based cut-off was finally achieved after multi-objective(or single-objective)optimization.A tradeoff was obtained to balance between the excessive workload of natural factors measurement in border fields and the low irrigation performance under the natural factor variations.The findings can provide a strong reference for border decisionmaking on the development of precision surface irrigation.