Efficient simulation of water-flow processes in the vadose zone is crucial to increase agricultural productivity within environmental limits. This requires deriving detailed soil hydraulic parameters of the soil profile that is highly challenging, particularly for heterogeneous soils. We therefore developed an alternative indirect methodology to calibrate the hydraulic parameters from soil water content time series measured at multiple depths by using the new physically based hydrological model HyPix.

We propose a novel, efficient, multistep optimization algorithm for layered soils that derives an optimal set of hydraulic parameters for a desired number of soil layers. For each selected soil layer, HyPix derives five physical, bimodal, Kosugi hydraulic parameters that describe the soil water retention and hydraulic conductivity by using a novel algorithm that reduces the degree of sensitivity and freedom of the parameters. The optimization algorithm upscales the soil hydraulic parameters by gradually incorporating the soil heterogeneity. This method overcomes the problems associated with optimization of the hydraulic parameters of each layer individually, which leads to poor results because it does not represent the cohesive soil water dynamics across the unsaturated zone.

We tested the method using soil water content measurements at different depths at five heterogeneous experimental sites in New Zealand. We show how the accuracy of the simulated water balance components increases with the number of soil layers. The multistep optimization upscales a detailed, layered profile of soil hydraulic parameters into a model with fewer layers. The methodology developed provides an estimate of the uncertainty of using a reduced number of soil layers. We also show that a pedological description can provide an indication of the minimum soil layers of vertical discretization required to accurately compute the soil water balance components.

对包气带中水流过程的有效模拟对于在环境限制内提高农业生产力至关重要。这需要导出土壤剖面的详细土壤水力参数，这是极具挑战性的，特别是对于异质土壤。因此，我们开发了一种替代的间接方法，通过使用新的基于物理的水文模型 HyPix 来校准在多个深度测量的土壤含水量时间序列的水力参数。

我们提出了一种新颖、高效、多步的分层土壤优化算法，该算法为所需的土壤层数推导出一组最佳的水力参数。对于每个选定的土壤层，HyPix 使用一种降低参数灵敏度和自由度的新算法，导出五个物理的、双峰的、Kosugi 水力参数，这些参数描述了土壤的保水性和导水率。优化算法通过逐渐结合土壤异质性来提升土壤水力参数。该方法克服了与单独优化每一层的水力参数相关的问题，由于它不能代表整个非饱和区的粘性土壤水动力学，因此导致结果不佳。

我们在新西兰的五个异质实验地点使用不同深度的土壤含水量测量值测试了该方法。我们展示了模拟水平衡组件的准确性如何随着土壤层数的增加而增加。多步优化将土壤水力参数的详细分层剖面升级为具有较少层的模型。所开发的方法提供了对使用减少的土壤层数的不确定性的估计。我们还表明，土壤学描述可以指示准确计算土壤水平衡分量所需的垂直离散的最小土壤层数。