Understanding spatial patterns of discharge in semi-arid regions using a recharge-discharge balance to determine vegetation health

[摘要] Executive Summary:Evapotranspiration, the flux of groundwater to the atmosphere through soil evaporation or vegetative transpiration, is one of the most difficult components of the water balance to estimate and model, but it is crucial for the prediction of salinisation.Accumulation of salt in a landscape due to rising water tables and increased rates of evapotranspiration is a significant environmental issue for Australia, with over 5.7 million hectares of land predicted to be at risk of salinisation in 2000.Since the 1930s when weirs were installed and irrigation commenced, salt accumulation has occurred in 25% of the South Australian floodplains of the lower River Murray, and has led to dieback of eucalypt vegetation communities.Although it is possible to predict risk of salinity longitudinally along the river on a large scale, the spatial patterns of groundwater discharge on the floodplains are not well understood.The objective of this study was to create a methodology to model long-term recharge and evapotranspiration processes in discharging catchments, at a scale appropriate for analysing and predicting vegetation health, and demonstrate its application for a lower River Murray floodplain.A methodology was developed using a spatial representation of plant water uptake and flooding using GIS, to generate input parameters for the groundwater flow model MODFLOW 2000.Using this methodology, long-term groundwater discharge and recharge rates were predicted for Clarks Floodplain, adjacent the Bookpurnong Irrigation District on the lower River Murray.This methodology enabled long-term recharge and discharge to be predicted with significantly more detail than using groundwater flow packages alone, and used a process with lower numerical complexity than fully coupled saturated unsaturated models.Results identified spatial patterns of groundwater discharge, including high areas of discharge associated with the break of slope and with E. camaldulensis forests at the ends of the floodplain peninsulas.Within these discharging areas the effect of floodplain elevation could also be seen.While the patterns of groundwater discharge were relatively stable for changes in the maximum evapotranspiration and recharge rate model inputs, variation in equilibrium depth caused some parts of the floodplain to change from long-term discharging areas to recharging areas.Sensitivity analyses also showed that variation in the recharge-discharge function on the floodplain did not significantly alter regional groundwater inflows to the system, but rather reapportioned water balance outflows between evapotranspiration, recharge and baseflow.Evapotranspiration and seepage at the break of slope in the modelled system was driven and limited by regional groundwater inflows, but ET nearer the river was driven by transpiration from vegetation communities on the floodplain peninsulas, and resourced by river water inflow.

[发布机构] CSIRO Land & Water

[学科分类] 地球科学(综合)