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Soil hydraulic properties of land units associated with the Oolloo Dolostone - Chapter 6

TitleSoil hydraulic properties of land units associated with the Oolloo Dolostone - Chapter 6
Publication TypeReport
Year of Publication2011
AuthorsHutley, LB, Northwood, M, Weinmann, R
Pagination64-90
Date Published12/2011
InstitutionDepartment of Natural Resources, Environment the Arts and Sport, 2011
CityDarwin, Northern Territory
Report NumberTechnical Report 22/2011D
ISBN Number978-1-921937-31-6
Abstract

Modelling surface water-groundwater interconnectivity requires data describing soil physical properties, vegetation water use, land use, topography and recharge processes. The relative contributions of surface runoff and drainage flux from the root zone to recharge needs to be quantified. One approach to archive this is to use biophysical models or SVAT models (soil-vegetation-atmosphere transfer). These models describe interactions between soil hydrological properties and depth, water table depth, vegetation characteristics and climate. A key component is the physical description of soil water holding capacity, the various forms of the soil moisture characteristic and unsaturated hydraulic conductivity. Most SVAT models use numerical solutions to solve the Richard’s equation for unsaturated flow within the profile and can simulate redistribution and recharge below the root zone to aquifer.  This enables water availability to vegetation to be calculated through a profile. Given water use, growth and root distribution of particular vegetation can be modelled as driven by climate, these models can be used to examine land use change scenarios and impacts on runoff and recharge processes.

Soil moisture dynamics are driven by soil water potential gradients developed by rainfall, surface evaporation, water uptake and drainage and SVAT models typically use soil hydraulic models such as those of Cambell and Shiozawa (1992) or van Genuchten et al. (1992) to describe both water holding properties plus saturated and unsaturated hydraulic conductivity as a function of water potential. There is a close coupling between soil available water and vegetation structure, particularly in savanna vegetation (Williams et al. 1996, Hutley et al. 2011), and modelling belowground processes (root distribution, growth, canopy development and water use and soil moisture distribution) is critical for the accurate simulation of soil-vegetationatmospheric transfer of water (evapotranspiration, ET). Robust simulation of soil water-vegetation dynamics results in accurate simulation of other components of the water balance such as runoff and drainage to aquifers.  This chapter aims to provide improved hydrological characterisation of land systems associated with the Oolloo region (the areal extent of the Oolloo Dolostone, Figure 1-1) that will enable more accurate modelling of surfacewater and groundwater connectivity via SVAT models (e.g. Chapter 7) for this groundwater system. While there has been extensive morphological descriptions and mapping of agriculturally important soil types across the Daly River catchment, there is little available data describing soil hydraulic functions and rainfall-runoff functions for all soil types. Data is available to describe rainfall-runoff relationships for agriculturally important red kandosol soils based on published functions of Dilshad et al. (1994, 1996b) but no data is available for land units with little agricultural potential that feature skeletal soils. These soils make up a significant fraction of the catchment and occur in upland, headwater areas and significant drainage and runoff is likely to arise from these land systems. If catchment scale modelling is to be precise enough to inform management decisions, hydraulic and runoff characteristics of these skeletal soils that occur in headwaters is required.

This chapter describes three aims to better describe surface hydraulics of land units associated with the Oolloo region, namely:

•    measure of soil hydraulic properties with a focus on soils of the Oolloo region with little or no soil physical data;

•    develop simple pedotransfer functions for the soil types sampled; and

•    develop rainfall-runoff functions for land units featuring skeletal soils.