ARS | Publication request: NEW MODELS FOR UNSATURATED SOIL HYDRAULIC PROPERTIES

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 12, 1994
Publication Date: N/A

Interpretive Summary: Computer models are being widely used in research and management to predict water flow and solute transport in the unsaturated zone between the soil surface and the groundwater table. The accuracy of the predictions depends greatly on the reliability of the hydraulic properties of the system being simulated. Especially important is the unsaturated hydraulic conductivity, K, which affects the rate and direction at which water and chemicals move through the unsaturated zone. Unfortunately, its measurement is difficult, costly, time- consuming, and frequently inaccurate. An alternative to direct measurements is the use of theoretical pore-scale models to predict K from the more easily measured soil water retention curve specifying the water content of soils under different soil dryness conditions. In this paper we propose relatively simple models for the water retention curve of both homogeneous and structured (macroporous) soils. The flexibility of the retention models and accuracy of the predicted conductivity models were tested against observed hydraulic data of more than 20 soils. Good agreement between predicted values and measured retention and conductivity data was found for most of the soils. The hydraulic models can be effectively used as inputs for numerical models simulating subsurface water flow and solute transport.

Technical Abstract: Two relatively simple models are proposed for describing the soil water retention curve. The expressions define sigmoidal or bimodal type retention functions with four or five parameters, respectively. The sigmoidal retention model may be combined with predictive pore-size distribution theories to yield closed-form equations for the unsaturated hydraulic conductivity. Parameters in the proposed hydraulic functions were estimated from observed retention data using a nonlinear least-squares mization process. The models were tested on hydraulic data for more than 20 soils. Good agreement between predicted values and measured retention and conductivity data was found for most of the soils. The soil hydraulic models can be effectively utilized as inputs for numerical models of water flow and solute transport.