Data underlying the publication: Hydrological consequences of controlled drainage with subirrigation

Controlled drainage with subirrigation (CD-SI) could be a viable measure to i) retain, ii) recharge, and iii) discharge recharge fresh water in agricultural fields. Four field CD-SI pilots with varying geohydrological conditions in the sandy Pleistocene uplands in the Netherlands were monitored (minimum 5 years) to study the effects on groundwater level, soil moisture content and soil water potential. Measurements include time series of groundwater level, soil moisture content, ditch level and CD-SI crest level, pit levels, and water supply. Field data were used for dynamic modelling with the agro-hydrological model Soil-Water-Atmosphere-Plant (SWAP). Calibrated SWAP models were used to i) model CD-SI systems dynamically and ii) model the hydrological consequences of subirrigation. Field measurements on four experimental plots showed that the water supply by to CD-SI systems can be high (ranging between roughly 500 mm to 1000 mm in the field sites), but CD-SI systems are able to raises the groundwater level such that soil water availability for crops increases. Furthermore, this study showed that CD-SI systems alter the hydrological fluxes significantly. Comparison of the four experimental fields also showed that a resistance to downward flow is needed to reduce downward seepage losses. Excessive downward seepage, or drainage losses towards surface water, increase the required water supply. However, unnecessary ditch drainage losses can be avoided by adapting the surface water level to the groundwater level. The data of these field experiments are required to understand the real-world situation better, leading to better models in terms of schematization, processes modelled, and model parameter values. This study showed that field pilots varying in geohydrological conditions could be modelled acceptably well with using SWAP. Both the required water supply and the water level in the control pit of the CD-SI system were simulated dynamically, which is a key element in understanding the functioning of CD-SI systems. The process-based model results lead to insight in the water balance components, also those components that cannot be (easily) measured in the field, and for in (extreme dry or wet) meteorological conditions that were not part of the experimental periods. Based on this research a number of recommendations are given to improve the implementation and operation of CD-SI systems.

带地下灌溉的控制性排水（Controlled drainage with subirrigation, CD-SI）可作为农业田块中实现（i）蓄水留存、（ii）地下水补给及（iii）排放补给淡水的可行措施。本研究针对荷兰更新世砂质高地中水文地质条件各异的4处CD-SI田间试点开展了不少于5年的监测，以探究其对地下水位（groundwater level）、土壤含水量（soil moisture content）与土壤水势（soil water potential）的影响。监测数据涵盖地下水位、土壤含水量、沟渠水位（ditch level）、CD-SI堰顶水位（CD-SI crest level）、测井水位（pit levels）及供水量（water supply）的时间序列。研究将田间实测数据应用于农业水文模型——土壤-水-大气-植物（Soil-Water-Atmosphere-Plant, SWAP）的动态建模。通过校准后的SWAP模型，本研究（i）动态模拟CD-SI系统，（ii）分析地下灌溉的水文效应（hydrological consequences）。
4处试验地块的田间实测结果显示，CD-SI系统的供水量可达到较高水平（田间站点供水量大致介于500 mm至1000 mm之间），且CD-SI系统可抬升地下水位，进而提升作物有效土壤水分供给能力。此外，本研究证实CD-SI系统会显著改变农田水文通量（hydrological fluxes）。对4处试验田的对比分析还表明，需设置向下水流阻力以减少向下渗漏损失（downward seepage losses）；过量向下渗漏或向地表水的排水损失（drainage losses towards surface water），会提升系统所需供水量。不过，通过将地表水位与地下水位相匹配，可避免不必要的沟渠排水损失。
此类田间试验数据有助于更深入地理解实际农田水文状况，进而在模型概化（schematization）、模拟过程（processes modelled）与模型参数取值层面优化模型。本研究表明，针对水文地质条件各异的田间试点，采用SWAP模型可实现较好的模拟效果。研究同时完成了CD-SI系统所需供水量与控制测井水位的动态模拟，这是理解CD-SI系统运行机制的核心要素。基于过程的模型模拟结果可帮助我们解析水量平衡各组分，包括难以在田间直接实测的组分，以及试验周期未覆盖的极端干旱或湿润气象条件（meteorological conditions）下的水量平衡组分。基于本研究成果，本文提出了若干优化CD-SI系统实施与运行的建议。