Abstract
The two-layered (0 - 50 and 50 - 250 mm) surface horizon hydraulic parameters of three dryland floodplain soil-types under aquafer water management in Postmasburg, Northern Cape Province of South Africa were estimated with HYDRUS-1D model. Time dependent water infiltration measurements at 30 and 230 mm depths from simulated rainfalls on undisturbed 1 m2 small plots with intensities of 1.61 (high), 0.52 (medium) and 0.27 (low) mm·min-1, were minimised using a two-step inversion. Firstly, separate optimisation of the van Genuchten-Mualem model parameters for the two surface-horizon layers and secondly, simultaneous optimisation for the joint two-layered horizon with first step optimal parameters entered as initial values. The model reproduced transient water-infiltration data very well with the Nash-Sutcliffe model efficiency coefficient (NSE) of 0.99 and overestimated runoff (NSE; 0.27 to 0.98). The upper surface horizon had highly optimised and variable parameters especially θs and Ks. Optimal Ks values from higher soil surface bulk-density (≥1.69 g·cm-3) were lower by at least one order of magnitude to double ring infiltrometers and water infiltration properties were different (P < 0.05) for the high rainstorm due to raindrop impact and surface crusting. Optimal α and n parameter values corresponded well with texture of the Addo (Greysols), Augrabies (Ferralsols) and Brandvlei (Cambisols) soil types. However, θs and Ksshowed greater sensitivity to model output and exerted greater influence on dryland floodplain water-infiltration and runoff characteristics. Increasing rainfall simulation period to attain near-surface saturated conditions and inclusion of surface ponding data in the inverse problem could considerable improve model prediction of hydro-physical parameters controlling surface-subsurface water distribution in fluvial environments.
Highlights
Soil hydraulic properties controlling infiltration and runoff play an important part in capturing and distributing water resources in dry riverbed and floodplains
Surface horizon hydraulic parameters controlling water infiltration and runoff of the van Genuchten-Mualem analytic model were estimated from three floodplain soil types using HYDRUS-1D model
A two-step inversion approach was used to estimate optimal parameter values for a two-layered surface horizon discretised at 0 - 50 mm and 50 - 250 mm depths
Summary
Soil hydraulic properties controlling infiltration and runoff play an important part in capturing and distributing water resources in dry riverbed and floodplains These fluvial environments are strategic sites for groundwater recharge and water-resource development. The SWRC represents the relationship between water content (θ) and metric suction (h), and mathematically represented by various pore-size distribution models ([3] [4] [5] [6]). These analytic models use parameter based closed-form equations to describe θ-h relationship and are used to predict other more difficult properties, K(h) or K(θ) in particular. Application of analytic models has been actively studied for several decades, but relatively few studies exist in which hydraulic parameters of fluvial soil types have been estimated using data from in situ experiments
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