基于根系层水分状态的旱区净灌溉需水量模型构建和应用

为更准确计算灌区净灌溉需水量，促进灌区水资源高效利用，针对降水过剩可能产生深层渗漏和地表径流，以及采用经验公式计算不同作物有效降水量可能错误估算净灌溉需水量的问题，该研究建立了基于根系层水分状态的净灌溉需水量模型。以景电灌区为例，计算了2000－2020年平均净灌溉需水量和有效降水量，并分析了各驱动因子对净灌溉需水量的影响，结果表明：不同作物年净灌溉需水量在319.4～732.3 mm之间，降水利用效率在39.2%～56.1%之间，夏秋作物的降水利用效率高于春夏作物。夏秋作物的年净灌溉需水量与年降水量相关性更强，春夏作物的年净灌溉需水量与作物需水量的相关性更强，所有作物的月净灌溉需水量仅与月作物需水量呈显著正相关。敏感性分析表明，净灌溉需水量与作物需水量呈正相关关系，与降水量和根系深度呈负相关关系。夏秋作物比春夏作物对降水量和作物需水量的敏感性更强。对净灌溉需水量贡献程度由大到小分别为作物需水量、降水量和根系深度，其中作物需水量贡献率占86.0%发挥主导作用，特定年份根系深度贡献率为12.0%，根系深度对净灌溉需水量的影响不容忽视。与传统净灌溉需水量模型相比，该研究所计算的净灌溉需水量充分考虑了不同作物降水利用效率的差异，计算结果可为灌区水资源管理提供参考。

Constructing and applying the net irrigation water requirement model for arid areas using water state of root zone

An accurate prediction of the net irrigation water demand can greatly contribute to promoting the efficient use of water resources in the irrigation areas. Empirical formulas are normally used to calculate the effective precipitation of different crops for the estimation of the net irrigation water demand. However, the excess precipitation may cause deep seepage and surface runoff, leading to incorrect prediction. In this study, a calculation model of net irrigation water demand was established using the three states of soil moisture in the root zone, including the water-deficient, residual water, and full-water state. The model also fully considered the deep percolation and surface runoff caused by precipitation. Five independent parameters were evaluated for the net irrigation water demand, including the precipitation, crop water demand, root depth, field capacity, and wilting point. Taking the Jingdian Irrigation District in Gansu Province of China as an example, the annual average and monthly average net irrigation water demand, and the effective precipitation were calculated to clarify the influence of three variables (precipitation, crop water demand, and root depth) on the net irrigation water demand. The results showed that the annual net irrigation water requirement of different crops was between 319.4 and 732.3 mm, while, the precipitation use efficiency was between 39.2% and 56.1%. Furthermore, the precipitation use efficiency of summer-autumn crops was higher than that of spring-summer ones. The annual net irrigation water demand of summer-autumn crops presented a stronger correlation with the annual precipitation, whereas, that of spring-summer crops shared a stronger correlation with the crop water demand. Monthly net irrigation water demand for all crops was significantly correlated with the monthly crop water demand only. The crop water demand was positively correlated with the net irrigation water demand, while the precipitation and root depth were negatively correlated with the net irrigation water demand. The sensitivity of crop water demand to annual net irrigation water demand was between 10.62% and 13.49% for the nine main crops, the sensitivity of precipitation to annual net irrigation water demand was between 1.00% and 4.76%, and the sensitivity coefficient of root depth to the most crops was 0, where the soybean and wolfberry showed the minor effects. Summer-autumn crops were more sensitive to the precipitation and crop water demand than the spring-summer crops, where all crops were less sensitive to the root depth. Overall, the variables contributing to the net irrigation water demand were ranked in descending order of the crop water demand, precipitation, and root depth, of which 86.0% of crop water demand was absolutely dominant. Specifically, the contribution rate of precipitation to the net irrigation water demand was between 8.1% and 25.9%, and that of crop water demand to the net irrigation water demand was between 73.5% and 91.4%. The contribution rate of precipitation to the net irrigation water requirement for the summer-autumn crops was greater than 15%, the contribution rate of spring-summer crops to the net irrigation water demand was greater than 85%, and the average contribution rate of root depth was only 0.2%. The contribution rate of root depth to the net irrigation water demand reached 12.0% from 2000 to 2020, indicating that the root depth posed a significant impact on the net irrigation water demand in specific years. In addition to annual precipitation, the temporal distribution of precipitation also posed the contribution of root depth to the net irrigation water demand during the year. Consequently, the net irrigation water demand model can be expected to fully consider the difference in the effective precipitation of different crops, compared with the traditional one. The finding can provide a strong reference for the water resource management in the irrigation areas.