Glen Walker

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.

Towards a framework for predicting impacts of land-use on recharge: 1. A review of recharge studies in Australia

This work investigated the potential for developing generic relationships from measurements of recharge made in previous studies that would allow the assessment of the impact of land-use change on recharge. Forty-one studies that measured recharge from across Australia were reviewed to generate a database. Studies were characterised on the basis of broad soil type (sand or non-sand), land-use/vegetation (annual, perennial, or trees), and annual rainfall. Attempts to develop quantitative recharge relationships met with limited success because of the limited geographical coverage of the studies, lack of details on the study sites, and high variability in the data. Nevertheless, the following relationships for annual vegetation were statistically valid: Ln(recharge) = –19.03 + 3.63 ln(rainfall) [for sandy soils]; F(1, 96) = 149.03; R 2 = 0.60 Ln(recharge) = –12.65 + 2.41 ln(rainfall) [for non-sandy soils]; F(1,151) = 46.87; R 2 = 0.23 The low degree of explanation of rainfall for the annual × non-sand data suggests that it is likely that soil structure becomes more important for higher clay content soils. Recharge under trees was negligible compared with that under annuals. These relationships should not be used in areas such as those where: preferential pathway flow is the dominant recharge mechanism, rainfall is summer dominant, lateral hydraulic gradients are high, water holding capacities are very low, or there are fresh, high-yielding aquifers. Collectively, the results show that: (1) rainfall explains a significant proportion of the observed recharge variation; (2) there is a significant difference between mean recharge under trees and annual vegetation; (3) there is a significant difference between mean recharge under annual vegetation on sand soils and non-sand soils; (4) the land-use groups had a greater influence on recharge than the broad soil groups used in this study; (5) there is a lack of annual recharge measurements under perennial pastures/crops, under trees in high rainfall zones (i.e. >600 mm/year) and in areas of summer dominant rainfall; (6) across a broad range of locations, recharge is higher under shallow-rooted annual vegetation than deep-rooted vegetation; and (7) the estimator of Zhang et al. (1999) for ‘excess water’ may provide a useful indication of the upper limit to the long-term average recharge measurements. Large variation in the data resulted from disparity in the recharge techniques used, the coarse soil categories used, failure to account for land management factors, and complications due to macropores and shallow water tables. It is proposed that the value of the information presented here may be enhanced in future studies by incorporating information from qualitative studies, particularly paired-site studies, and by drawing information from unsaturated zone and groundwater modelling studies, particularly comparisons of different land-uses at similar locations. Furthermore, the results of this study can be used to identify gaps in knowledge and, hence, target areas for future research such as annual recharge measurements beneath perennial vegetation. Additional keywords: dryland salinity, groundwater management, deep drainage.

Unsaturated zone tritium and chlorine 36 profiles from southern Australia: Their use as tracers of soil water movement

In this paper we present seven unsaturated zone profiles of 36Cl and three 3H profiles from southern Australia. All profiles show single peaks corresponding to high radionuclide fallout from nuclear testing in the 1950s and 1960s. The profiles are used to estimate rates of water movement leading to recharge of the groundwater. Among these profiles is the first profile on which high concentrations of 36Cl have been found below a 2-m depth. In this profile, 3H and 36Cl peaks coincide. In six of the seven profiles, total 36Cl fallout was found to be between 1.2 and 2.4 × 1012 atoms m−2, and is of similar magnitude to that found at similar latitudes in the northern hemisphere. Comparisons of soil water fluxes estimated with 3H, 36Cl, and chloride are briefly discussed.

The Source of Water Transpired by Eucalyptus camaldulensis: Soil, Groundwater, or Streams?

Publisher Summary This chapter describes the relative importance of various possible sources of water available to river red gums or Eucalyptus camaldulensis. The technique used in this study monitored the stable isotope composition of water from tree sap and different possible water sources. However, a detailed analysis of isotope data and measurement of soil water status was used to provide insights into the water relations of Eucalyptus camaldulensis. The study not only addresses an important ecological problem but also provides an example of the power of isotopic methods for studying plant-water sources. Naturally occurring differences in the concentration of the stable isotopes D and 18 O were used to determine the sources of water transpired by E. camaldulensis. The comparison of tree sap and creek water δD and δ 18 O data showed that creek water is not the dominant source of water for trees flooded by creek water. The combination of the δD and δ 18 O data allowed application of a simple two end-member mixing model to estimate the proportion of groundwater and shallow soil water used by the trees.

The movement of isotopes of water during evaporation from a bare soil surface

Stable isotopes of water (2H, 18O) at natural levels have been used to measure steady-state evaporation from soil surfaces in situations where other techniques would fail. In an endeavour to generalise the technique, we have investigated the movement of isotopes during nonsteady evaporation, especially during second-stage evaporation. Equations describing the isotope profiles are developed and it is found that the isotope concentration is a function of the Boltzmann variable (depth time−12). The equations are then solved under the assumption that the evaporation front is narrow. Seven packed sand columns were allowed to evaporate under controlled conditions and then sectioned for analysis of water content and isotopic composition. The results agree generally with the theory but there are difficulties with using isotopes to estimate cumulative evaporation.

Impact of flooding on the water use of semi-arid riparian eucalypts

The water use strategy of Eucalyptus largiflorens (F. Muell.) was investigated in response to flooding on the Chowilla Anabranch, a semi-arid floodplain of the Murray River, South Australia. Water use was measured using the heat pulse technique at six sites that varied in flood duration from 0 to 78 days. Soil chloride, plant water potential and surface root mass were also measured. Suppression of tree water use did not occur during flooding regardless of flood length and site health, suggesting that sufficient oxygen had been available to the trees. Increases in tree water use occurred at some sites after the flood because of increases in water availability due to leaching of salt from the soil profile. The soils with a higher clay content incurred little leaching of salts and therefore little change in tree water availability. In contrast, the sites with more sandy soils encountered greater leaching and greater increases in tree water availability. Despite differing soil type responses, all tree communities investigated showed a reduction in tree water stress in the period after flooding. These findings suggest that flooding in this environment improves the health of Eucalyptus largiflorens in the short-term. The implications of these findings are discussed with regard to the management of the Chowilla Anabranch.

Tree water sources over shallow, saline groundwater in the lower River Murray, south-eastern Australia: implications for groundwater recharge mechanisms

The decline of riparian vegetation in the lower River Murray, south-eastern Australia, is associated with a reduction in flooding frequency, extent and duration, and increased salt accumulation. The plant water sources of healthy Eucalyptus largiflorens trees growing over highly saline (>40 dS m −1 ) groundwater were investigated during summer when water deficit is greatest. The study found low-salinity soil water overlying highly saline groundwater at most sites. This deep soil water, rather than the saline groundwater, was identified as the plant water source at most sites. Stable isotopes of water and water potential measurements were used to infer how the deep soil water was recharged. The low-salinity, deep soil water was recharged in the following two ways: (1) vertically through the soil profile or via preferential flow paths by rainfall or flood waters or (2) horizontally by bank recharge from surface water on top of the saline groundwater. Vertical infiltration of rainfall and floodwaters through cracking clays was important for trees growing in small depressions, whereas infiltration of rainfall through sandy soils was important for trees growing at the break of slope. Bank recharge was important for trees growing within ∼50 m of permanent and ephemeral water bodies. The study has provided a better understanding of the spatial patterns of recharge at a scale relevant to riparian vegetation. This understanding is important for the management of floodplain vegetation growing in a saline, semi-arid environment.

Modelling vegetation health from the interaction of saline groundwater and flooding on the Chowilla floodplain, South Australia

The native riparian vegetation communities on the Chowilla floodplain in the lower River Murray in South Australia are suffering severe declines in health, particularly the Eucalyptus camaldulensis Dehnh. (red gum) and Eucalyptus largiflorens F.Muell. (black box) communities. The primary cause of the decline is salinisation of the floodplain soils caused by increased rates of groundwater discharge and hence increased movement of salt up into the plant root zone. The salinity is driven by a lack of flooding and rising saline groundwater tables. Rises in the naturally saline groundwater levels are due to the effects of river regulation from Lock 6 and high inflows from regional groundwater levels increased by Lake Victoria to the east. River regulation has also led to reduced frequency and duration of the floods that leach salt from the plant root zone and supply fresh water for transpiration. The frequency of medium-sized floods occurring on Chowilla has been reduced by a factor of three since locking and water extractions were commenced in the 1920s to provide reliable water for urban and agricultural use. The soil salinisation on the floodplain was modelled by using a spatial and temporal model of salt accumulation from groundwater depth, groundwater salinity, soil type and flooding frequency. The derived soil water availability index (WINDS) is used to infer vegetation health and was calibrated against current extent of vegetation health as assessed from fieldwork and satellite image analysis. The modelling work has shown that there is a severe risk to the floodplain vegetation from current flow regimes. This paper estimates that 65% (5658 ha) of the 8600 ha of floodplain trees are affected by soil salinisation matching a field survey of vegetation health in 2003 (Department of Environment and Heritage 2005a), compared with 40% in 1993 (Taylor et al. 1996). Model results show that the best management option for Chowilla is lowering the groundwater down to 2 m below current levels, which predicts an improvement in the health of the floodplain tree species from 35 to 42%.

Episodic recharge and climate change in the Murray-Darling Basin, Australia

In semi-arid areas, episodic recharge can form a significant part of overall recharge, dependant upon infrequent rainfall events. With climate change projections suggesting changes in future rainfall magnitude and intensity, groundwater recharge in semi-arid areas is likely to be affected disproportionately by climate change. This study sought to investigate projected changes in episodic recharge in arid areas of the Murray-Darling Basin, Australia, using three global warming scenarios from 15 different global climate models (GCMs) for a 2030 climate. Two metrics were used to investigate episodic recharge: at the annual scale the coefficient of variation was used, and at the daily scale the proportion of recharge in the highest 1% of daily recharge. The metrics were proportional to each other but were inconclusive as to whether episodic recharge was to increase or decrease in this environment; this is not a surprising result considering the spread in recharge projections from the 45 scenarios. The results showed that the change in the low probability of exceedance rainfall events was a better predictor of the change in total recharge than the change in total rainfall, which has implications for the selection of GCMs used in impact studies and the way GCM results are downscaled.RésuméDans les régions semi arides la recharge épisodique peut constituer une partie importante de la recharge totale dépendant d’épisodes pluvieux rares. Avec des prévisions de changement climatique suggérant de futurs changements d’ampleur et d’intensité des précipitations, la recharge de nappe dans les régions semi arides sera vraisemblablement affectée de façon disproportionnée par le changement climatique. Cette étude cherchait à examiner les changements liés à une recharge épisodique dans les zones arides du Murray-Darling Basin, Australie, en utilisant trois scénarios de réchauffement global tirés de 15 modèles différents (GCMs) du climat 2030. Deux grandeurs ont été utilisées pour examiner la recharge épisodique: le coefficient de variation à l’échelle de l’année, et le pourcentage de recharge au plus haut 1% du jour à l’échelle journalière. Les grandeurs étaient proportionnelles mais ne permettaient pas de conclure si la recharge épisodique devait ou non augmenter dans cet environnement; ceci n’est pas un résultat surprenant étant donnée la dispersion des projections de recharge des 45 scénarios. Les résultats ont montré que le changement dans la probabilité basse d’un excédent des événements pluvieux était un meilleur indicateur du changement de recharge totale que le changement de précipitation totale, ce qui a des implications dans la sélection des GCM utilisés dans les études d’impact et la façon dont les résultats du GCM sont réduits.ResumenEn áreas semiáridas la recarga episódica puede formar una parte significativa de la recarga total, dependiendo de los eventos infrecuentes de las precipitaciones. Con las proyecciones de cambio climático sugiriendo cambios en las intensidades y magnitudes de las futuras precipitaciones, probablemente la recarga del agua subterránea en área semiáridas se verá afectada desproporcionadamente por el cambio climático. Este estudio buscó investigar los cambios proyectados en la recarga episódica en áreas áridas de la cuenca Murray-Darling, Australia, usando tres escenarios de calentamiento global a partir de 15 diferentes modelos climáticos globales (GCMs) para el clima en 2030. Se utilizaron dos métricas para investigar la recarga episódica: en escala anual se usó el coeficiente de variación, y en la escala diaria la proporción de la recara en un 1% más alto de la recarga diaria. Estas métricas eran recíprocamente proporcionales pero no fueron concluyentes acerca si la recarga episódica iba a incrementarse o disminuir en este ambiente; esto no es un resultado sorprendente considerando la amplitud en las proyecciones de la recarga a partir de 45 escenarios. Los resultados mostraron que el cambio en la baja probabilidad de eventos de excedencia de las precipitaciones fue un mejor predictor del cambio en la recarga total, lo cual tiene implicancias para la selección de GCMs usados en estudios de impacto y la manera en que los resultados del GCM son llevados a escalas más reducidas.摘要在半干旱地区,间歇性补给量能占到总补给量的很大一部分,取决于不频发的降雨事件。由于气候变化会导致未来降雨大小和强度的变化,半干旱地区的地下水补给很可能不成比例地受气候变化影响。本研究根据从15个不同的全球气候模型(GCMs)中选择的3个全球暖化情景,分析了2030年的气候条件,调查了澳大利亚Murray-Darling盆地干旱地区地下水间歇性补给的变化。利用两个变量来调查间歇性补给:年尺度上的变量系数以及日尺度上日补给量最高1%的补给比例。变量二者之间是成比例的,但是这个环境条件下间歇性补给量到底是增加还是降低的,则是不确定的,考虑到45个情景中补给变化的广度,这个结果并不奇怪。结果表明超过的降雨事件的低概率变化相比总的降雨变化,是更好地总补给量变化的预测方法,这对影响研究中全球气候模型的选择以及该模型结果的尺度缩小方法是非常有意义的.ResumoA recarga episódica em zonas semi-áridas pode constituir uma parte significativa da recarga total, dependente de eventos de precipitação raros. Com as projeções para as alterações climáticas a sugerir futuras modificações na grandeza e na intensidade da precipitação, é provável que a recarga da água subterrânea em zonas semi-áridas venha a ser afectada desproporcionadamente pelas alterações climáticas. Este estudo procurou investigar as alterações projectadas da recarga episódica em zonas áridas da Bacia de Murray-Darling, Austrália, usando três cenários de aquecimento global em 15 diferentes modelos climáticos globais (GCM) para um clima em 2030. Foram usadas duas métricas para investigar a recarga episódica: à escala anual foi usado o coeficiente de variação e à escala diária usou-se a proporção de recarga das maiores percentagens de recarga diária. As métricas foram proporcionais entre si, mas foram inconclusivas quanto à possibilidade da recarga episódica ir aumentar ou diminuir neste ambiente, o que não é um resultado surpreendente, considerando a dispersão das projeções de recarga a partir dos 45 cenários. Os resultados mostraram que as mudanças na baixa probabilidade de superação de eventos de precipitação foi um melhor preditor das modificações na recarga total, do que a mudança na precipitação total, o que tem implicações para a selecção dos GCM utilizados em estudos de impacte e na forma como os resultados de GMC são aplicados localmente.

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).