Spatiotemporal sampling strategy for characterization of hydraulic properties in heterogeneous soils
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ORIGINAL PAPER
Spatiotemporal sampling strategy for characterization of hydraulic properties in heterogeneous soils Danyang Yu1 • Yuanyuan Zha1,2
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Liangsheng Shi1 • Andrei Bolotov3 • Chak-Hau Michael Tso4,5
Accepted: 23 September 2020 Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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. Keywords Variably saturated flow Soil heterogeneity Cross-correlation analysis Sampling strategy Data assimilation
1 Introduction Soil hydraulic properties are essential for accurate predictions of water movement and solute distributions in the vadose zone (Vereecken et al. 2016). However, detailed knowledge of soil properties is challenging, since the direct Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00477-020-01882-1) contains supplementary material, which is available to authorized users.
measurement of hydraulic and pneumatic flow parameters of small-scale samples at a large number of locations is time-consuming, costly, and impractical (Huang et al. 2009; Illman and Tartakovsky 2005; Yeh and Liu 2000). Nowadays, datastream of the soil hydraulic states (e.g., soil moisture and pressure head) can be collected conveniently via in-situ sensors. Incorporating these observations into vadoze zone flow models helps to reduce
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