Warrick Dawes

Response of mean annual evapotranspiration to vegetation changes at catchment scale

It is now well established that forested catchments have higher evapotranspiration than grassed catchments. Thus land use management and rehabilitation strategies will have an impact on catchment water balance and hence water yield and groundwater recharge. The key controls on evapotranspiration are rainfall interception, net radiation, advection, turbulent transport, leaf area, and plant-available water capacity. The relative importance of these factors depends on climate, soil, and vegetation conditions. Results from over 250 catchments worldwide show that for a given forest cover, there is a good relationship between long-term average evapotranspiration and rainfall. From these observations and on the basis of previous theoretical work a simple two-parameter model was developed that relates mean annual evapotranspiration to rainfall, potential evapotranspiration, and plant-available water capacity. The mean absolute error between modeled and measured evapotranspiration was 42 mm or 6.0%; the least squares line through the origin had as lope of 1.00 and a correlation coefficient of 0.96. The model showed potential for a variety of applications including water yield modeling and recharge estimation. The model is a practical tool that can be readily used for assessing the long-term average effect of vegetation changes on catchment evapotranspiration and is scientifically justifiable.

Estimation of soil moisture and groundwater recharge using the TOPOG_IRM Model

A distributed parameter ecohydrological model (TOPOG_IRM) was applied to a 1.6 km2 pastoral catchment in southeast Australia for estimating soil moisture and groundwater recharge, for a period of 1992–1994. The main objective of TOPOG_IRM is to provide a realistic description of the key processes which control the soil moisture dynamics, evapotranspiration, and to investigate hydrological and ecological responses at a catchment scale. The model was not fitted but parameterised with field data and literature values. The soil moisture content at various depths was well simulated for six sites in the catchment representing different soil types and landscape positions. The spatial patterns of the soil moisture content appear to be controlled mainly by the soil types, and the results indicated that there is little lateral movement of water in the catchment. Average groundwater recharge was found to be 5% of the annual rainfall, and its spatial patterns were dominated by the soil types. The study has demonstrated potential application of TOPOG_IRM in simulating catchment scale responses under different land management options in the context of salinity control.

Predicting the effects of landuse change on water and salt balance—a case study of a catchment affected by dryland salinity in NSW, Australia

An integrated and comprehensive framework for the assessment of water and salt balance for large catchments is presented. The framework is applied to the Mandagery Creek catchment (1688 km2), located in the south-eastern part of Australia. The catchment is affected by dryland salinity and the effects of landuse, climate, topography, soils and geology on water and salt balance are examined. Landuse change scenarios designed to: (a) increase the perennial content of the pastures and crop rotations and (b) increase the current remnant native woody vegetation with additional tree cover are investigated to determine the level of intervention required to develop ameliorative strategies. Likely downstream impacts of the reduction in water flow and salt export are also estimated.

Estimating episodic recharge under different crop/pasture rotations in the Mallee region. Part 2. Recharge control by agronomic practices

Abstract Much environmental degradation, including salinity in the Mallee region of southeastern Australia, is associated with the loss of native vegetation and increased recharge. As a result, various agronomic practices have been proposed to reduce groundwater recharge. This study was conducted to evaluate the impact of these practices on recharge, in particular episodic recharge. A biophysically based model (WAVES) was used to estimate recharge rates under some typical crop and pasture rotations in the region using long-term meteorological data. Results show that: (1) recharge just below the root zone was episodic and that just 10% of annual recharge events contributed over 85% of long-term totals. Management options such as incorporating lucerne and deep-rooted non-fallow rotations can reduce both, mean annual recharge, and the number of episodic events, but not eliminate recharge completely; (2) winter fallows increased soil-water storage and some of the additional water was stored in the lower portion of the root zone or below it. This can increase the risk of recharge to groundwater system; (3) changes in land management may take a considerable period of time (>10 years) to have any noticeable impacts on recharge; and (4) recharge under lucerne was ≈30% of that under medic pasture.

The significance of topology for modeling the surface hydrology of fluvial landscapes

The realistic representation of terrain in hydrological models of fluvial landscapes poses problems particularly for representing convergence and divergence of lateral flows. In this work we consider the topology of contour representation of surfaces. The networks of ridge and drainage lines are linked at critical points: peaks, saddles, confluences, and simple ridge junctions. The properties of these points are given, and from these, three topologic rules are derived. These are used to develop an automatic procedure for generating a catchment boundary and a modified flow net. Use of the flow net increases speed by 2 orders of magnitude over earlier contour-based topographic analyses, and the use of topology increases speed, accuracy, and sophistication over a recent flow net technique. Topology is used to evaluate simplifications of natural surfaces and, in particular, shows the unsuitability of the common ridge-to-stream rectangular flow strip. Finally, implications for the structure of distributed catchment models and for designing and interpreting field experiments are considered.

The strategic landscape investment model: a tool for mapping optimal environmental expenditure

This paper presents the strategic landscape investment model (SLIM). This tool can be used to map optimal landscape treatment patterns at regional scales. Developed for New South Wales (NSW) in Australia, SLIM aims to maximise an indexed measure of environmental benefit within a budget constraint. The attributes considered include salinity, water yield, nitrogen run-off, phosphorus run-off, stream sediment concentrations, soil erosion and carbon sequestration. The modelling is undertaken spatially with a roughly 1 km^2 grid covering NSW. With estimates of costs and benefits, maps of marginal environmental benefit per dollar expended can be constructed. These maps are used to define an optimal treatment pattern within the confines of a program budget. SLIM is demonstrated through an analysis of perennial pasture establishment on NSW grazing lands. It was found that the optimal treatment area is around 4% of the total treatable area, demonstrating the importance of careful investment targeting. Through sensitivity analysis it is found that the location of optimal landscape treatment patterns is relatively robust under numerous attribute weighting scenarios. The paper explores the strengths and weaknesses of SLIM considering how improved analytic capabilities could be added to future revisions.

Assessing the viability of recharge reduction for dryland salinity control: Wanilla, Eyre Peninsula

The emerging paradigm to manage the spread of dryland salinity is the manipulation of farming practice to provide both a reduction in recharge and a commercial return to farm enterprises. Recent work has attempted to classify the groundwater systems across Australia into distinct provinces, with the implication that the flow processes, and therefore remediation strategies, of catchments within each province are similar. This paper presents a case study of the Wanilla catchment on the Eyre Peninsula in South Australia. This catchment is in the groundwater province that includes 60% of the dryland salinity expression in Australia. The results of conceptual and numerical modelling of the catchment suggest that the land management for reduced recharge paradigm may be less effective in this groundwater province than in others. The scale of expression and salinity history of such catchments provides further impediments to management options aimed at controlling or reversing existing dryland salinity.

Biophysical modelling of catchment-scale surface water and groundwater response to land-use change

The effect of land-use change on salt and water-balances of catchments in Australia has been significant. Impacts of these changes are often masked by large time lags between the changes and their subsequent expression. Successful management relies on information that allows these changes to be understood and predicted. In the absence of detailed hydrogeological and hydrographic data, a simple approach is required. A logistic function model is introduced, which weights changes in recharge to changes in discharge according to a characteristic time-scale and a rate of change. This response function approach is used to estimate the time lags for individual groundwater flow systems (GFS), which can be aggregated to estimate whole catchment behaviour. Using this model, the predicted future effect of a range of afforestation strategies on catchment salt load has been simulated for sub-catchments of the mid-Macquarie (New South Wales, Australia).

Prioritisation approach for estimating the biophysical impacts of land-use change on stream flow and salt export at a catchment scale

Predicting the impacts of land-use change on stream flow and stream salt export at a catchment scale is hampered by limited detailed measured data, particularly with regard to hydrogeological information. A recently developed modelling approach is presented that can be used to predict the variation in likely catchment response to changes in woody cover using only broadly available data. The Biophysical Capacity to Change (BC2C) model combines a downward approach for water balance, with groundwater response using groundwater flow systems (GFS) mapping to provide hydrogeological and salinity parameters, into a spatial model for estimating the impacts of changes in woody vegetation cover across large areas. The results from the model are compared to gauged flow and salinity data for 14 stream gauging stations across the Murrumbidgee catchment, in south-eastern Australia. Considering the limited calibration of the model, the results compare favourably in broad terms, and provide a useful starting point for consideration of the impacts of land-use change on stream flow and salt load, and to guide catchment managers towards areas where more detailed study can be undertaken.

Simulation of winter wheat yield and water use efficiency in the Loess Plateau of China using WAVES

Abstract Water availability is a major factor that limits grain yield in the Loess Plateau of China. As competition for water intensifies between the agricultural and industrial sectors, it is essential to improve the agricultural water use efficiency (WUE) in the region. This requires the development of alternative irrigation schedules that maximize WUE. A field experiment was conducted for winter wheat ( Triticum aestivum L.) during the period 1995–1998 to calibrate and test a biophysically based model WAVES in terms of grain yield and WUE predictions. Different irrigation treatments were carried out and the data collected included water and energy balance components, biomass, and grain yield. The model was run for 3 years with the measured meteorological data and grain yield was modeled using two different formulations. Comparisons between the measurements and the model predictions were made with 3 years of independently measured data. The results showed that the simulated grain yield and WUE based on biomass and harvest index are in better agreement with the measurements than those based on transpiration and harvest index. The model is sensitive to different irrigation treatments, and in reasonable agreement with field measured data. Results from both the field experiments and model simulation showed that evapotranspiration was greatest in the highest irrigation treatment, but the grain yield was not the highest and WUE was relatively low. Appropriately limited irrigation could improve the grain yield and WUE. Therefore, the WAVES model can be used to simulate grain yield and WUE of winter wheat in the Loess Plateau. A better irrigation scheme should aim for maximum WUE and any attempts to solely increase grain yield in the region would not be sustainable.