Accurate characterization and prediction of soil moisture distribution and solute transport in vadose zone require detailed knowledge of the spatial distribution of soil hydraulic properties. Since the direct measurements of these unknown properties are challenging, many studies invert the soil hydraulic parameters by incorporating observation data (e.g., soil moisture and pressure head) at selected point sampling locations into soil moisture flow models. However, a cost-effective sampling strategy for where and when to collect the data, which is vital for saving the costs for monitoring and data interpretation, is relatively rare compared to the direct parameter retrieving efforts. Here, an optimal spatial–temporal sampling strategy was proposed based on cross-correlation analysis between observed state variables and soil hydraulic parameters. Besides, the effects of meteorological condition, observation type, bottom boundary condition, and correlation scale of soil hydraulic parameters are also demonstrated. The proposed sampling strategy was assessed by both synthetic numerical experiments and a real-world case study. Results suggest the retrieval accuracy of heterogeneous soil is acceptable if the spatial/temporal sampling interval is set to be one spatial/temporal correlation length of soil moisture. Besides, surface observation contains the most plentiful information which could be used to derive root-zone soil moisture/parameters, but this ability depends on the correlation scale of soil hydraulic parameters. Besides, the temporal value of soil moisture depends on meteorological condition. It is not necessary to sample repeatedly during dry periods, but more attention should be paid to the observations after rainfall events.

在渗流带中，准确表征和预测土壤水分分布和溶质运移需要详细了解土壤水力特性的空间分布。由于对这些未知特性的直接测量具有挑战性，因此许多研究通过将选定点采样位置处的观测数据（例如，土壤湿度和压头）合并到土壤水分流模型中来反转土壤水力参数。但是，与直接检索参数相比，对于在何处，何时何地收集数据而言具有成本效益的采样策略相对稀少，这对于节省监视和数据解释的成本至关重要。在此，基于观测状态变量与土壤水力参数之间的互相关分析，提出了一种最佳的时空采样策略。此外，还演示了气象条件，观测类型，底部边界条件和土壤水力参数的相关标度的影响。拟议的抽样策略通过综合数值实验和实际案例研究进行了评估。结果表明，如果将空间/时间采样间隔设置为土壤水分的一个空间/时间相关长度，则异质土壤的恢复精度是可以接受的。此外，地表观测还包含最丰富的信息，可用于得出根区土壤水分/参数，但这种能力取决于土壤水力参数的相关范围。此外，土壤水分的时间价值取决于气象条件。干燥期间无需重复采样，