Modelling the effects of climate and land cover change on groundwater recharge in south-west Western Australia
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Abstract:
Abstract. The groundwater resource contained within the sandy aquifers of the Swan Coastal Plain, south west Western Australia, provides approximately 60% of the drinking water for the metropolitan population of Perth. Rainfall decline over the past three decades coupled with increasing water demand from a growing population has resulted in falling dam storage and groundwater levels. Projected future changes in climate across south-west Western Australia consistently show a decline in annual rainfall of between 5 and 15%. There is expected to be a continuing reduction of diffuse recharge across the Swan Coastal Plain. This study aims to quantify the change in groundwater recharge in response to a range of future climate and land cover patterns across south-west Western Australia. Modelling the impact on the groundwater resource of potential climate change was achieved with a dynamically linked unsaturated/saturated groundwater model. A Vertical Flux Manager was used in the unsaturated zone to estimate groundwater recharge using a variety of simple and complex models based on land cover type (e.g. native trees, plantation, cropping, urban, wetland), soil type, and taking into account the groundwater depth. These recharge estimates were accumulated on a daily basis for both observed and projected climate scenarios and used in a MODFLOW simulation with monthly stress periods. In the area centred on the city of Perth, Western Australia, the patterns of recharge change and groundwater level change are not consistent spatially, or consistently downward. In the Dandaragan Plateau to the north-east of Perth there has been groundwater level rise since the 1970s associated with land clearing, and with rainfall projected to reduce the least in this area the groundwater levels are estimated to continue to rise. Along the coastal zone north of Perth there is an interaction between projected rainfall decline and legislated removal to pine forests. This results in areas of increasing recharge and rising water levels into the future despite a drying climate signal. To the south of Perth city there are large areas where groundwater levels are close to the land surface and not expected to change more than 1m upward or downward over the next two decades; it is beyond the accuracy of the model to conclude any definite trend. In the south western part of the study area, the patterns of groundwater recharge are dictated primarily by soil, geology and land cover. In the sandy Swan (northern boundary) and Scott Coastal Plains (southern boundary) there is little response to future climates, because groundwater levels are shallow and much rainfall is rejected recharge. The profile dries out more in summer but this allows more rainfall to infiltrate in winter. Until winter recharge is insufficient to refill the aquifers these areas will not experience significant falls in groundwater levels. On the Blackwood Plateau however, the combination of native vegetation and clayey surface soils that restrict possible infiltration and recharge mean the area is very sensitive to climate change. With low capacity for recharge and low storage in the aquifers, small reductions in recharge can lead to large reductions in groundwater levels. In the northern part of the study area both climate and land cover strongly influence recharge rates. Recharge under native vegetation is minimal and is relatively higher where grazing and pasture systems have been introduced after clearing of native vegetation. In some areas the low recharge values can be reduced to almost zero, even under dryland agriculture, if the future climate becomes very dry. In the Albany Area the groundwater resource is already over allocated, and the combination of existing permanent native vegetation with decreasing annual rainfall indicate reduced recharge. The area requires a reduction in groundwater abstraction to maintain the sustainability of the existing resource.Keywords:
MODFLOW
Land Cover
Depression-focused recharge
지하수 함양량은 기후조건, 토지이용, 수리지질학적 비균질성에 의해 시공간적인 변동성을 나타내는 수문량이므로 통합지표수-지하수 모델 기반의 시공간변동성을 갖는 일단위 함양량의 추정이 필요하다. 진천지역을 대상으로 SWAT-MODFLOW 통합모형이 일단위 함양량 추정에 사용되었으며 추정된 함양량의 시변성은 국가 지하수 관측망과 기초조사 기간(2009-2010)중에 설치된 자동관측망 자료와 잘 부합하는 것을 확인하였다. 진천지역을 포함한 미호천 유역 평균 지하수 함양률은 강수대비 20.8%로 나타났는데 이는 해석적 방법인 기저유출 분리법의 결과와도 잘 일치하였다. 통합모델링 기반의 함양량 산정은 국가 지하수 관리를 위해 유용하게 활용될 수 있을 것으로 판단된다. Because groundwater recharge shows spatial-temporal variability due to climatic conditions, it is necessary to investigate land use and hydrogeological heterogeneity, and estimate the spatial variability in the daily recharge rate based on an integrated surface-groundwater model. The integrated SWAT-MODFLOW model was applied to compute physically based daily groundwater recharge in the Jincheon region. The temporal variations in estimated recharge were calibrated using the observed groundwater head at several National Groundwater Monitoring Stations and at automatic groundwater-monitoring sites constructed during the Basic Groundwater Investigation Project (2009-2010). For the whole Mihocheon watershed, including the Jincheon region, the average groundwater recharge rate is estimated to be 20.8% of the total rainfall amount, which is in good agreement with the analytically estimated recharge rate. The proposed methodology will be a useful tool in the management of groundwater in Korea.
MODFLOW
Groundwater model
Depression-focused recharge
Groundwater discharge
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Depression-focused recharge
Infiltration (HVAC)
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Depression-focused recharge
Groundwater model
Groundwater discharge
Return flow
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MODFLOW
Depression-focused recharge
Groundwater model
Groundwater discharge
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Recharge can take two main forms, direct recharge from rainfall infiltrating the ground or
indirect recharge from leakage from wadi beds. The recharge processes operating in the West
Bank can be summarised as rainfall recharge, wadi recharge, urban recharge processes and
irrigation losses. Rainfall recharge is the predominant form of recharge, whilst wadi recharge,
urban and irrigation losses are only minor components. However, these minor components
can be locally important.
The recharge processes operating in the Wadi Natuf catchment are varied and complex. The
four main geological strata through which recharge takes place are:
• Jerusalam
• Upper Lower Beit Kahil
• Lower Beit Kahil
• Hebron
The main aquifer units are karstic which receive recharge once a wetting threshold is
exceeded. This assumption is supported by field observations (Messerschmid, 2003) and a
field experiment close to the study area (Lange et al., 2003). Other minor aquifers receive
recharge and distribute water laterally to springs. Flow from springs, if not used for water
supply or irrigation, can then be routed to other aquifer units or as loss from wadis.
High intensity rainfall can produce overland runoff and wadi flow. Flowing wadis loose
water to all but the Yatta formation.
Recharge can, therefore, occur by two methods, direct infiltration from rainfall and from
losses from wadi beds.
There are four main recharge processes operating in the aquifers of the West Bank;
1. Direct recharge from rainfall
2. Indirect recharge from wadi losses
3. Recharge from urban water supply and waste water proceses
4. Recharge from irrigation losses
The difference between rainfall and potential evaporation, known as effective rainfall, is the
main control on direct recharge from rainfall. Rainfall is greatest in the north and west
whereas potential evaporation is the highest in the south and east. The greatest potential for
rainfall recharge is, therefore, in the north and west. Soil cover also controls the amount of
rainfall recharge and is highly variable over the West Bank. In particular, the main soil types
have patchy coverage, over only 30-50 % of the ground surface, the rest being bare rock. The
patchiness of the soil means that soil moisture is not developed in the same way as for soils
with uniform coverage.
To determine the rainfall recharge mechanisms operating in the West Bank, a combination of
factors such as rainfall, potential evaporation, soil cover, land use, etc need to be assessed.
Combining these factors mean that recharge processes based on soil moisture are most likely
to be operating in the north-west of the West Bank. Elsewhere, direct recharge will be based
on how the soil and rocks combined as single system respond to the balance between rainfall
and evaporation (e.g. Lange et al., 2003). Indirect recharge occurs due to wadi flows over the whole of the West Bank. Runoff from
intense rainfall events will collect in valley bottoms and create surface water flows. Recharge
from wadi beds will form the predominant source of recharge in the south and east of the
West Bank, where the climate is more arid.
Urban recharge processes reflect leakage from pipes and sewers and increased runoff from
paved surfaces, roofs, roads, etc. The enhanced runoff in the urban environment is routed to
wadis and enhances flows after rainstorms. This can increase indirect recharge from wadi
beds.
Losses from irrigation systems can enhance recharge. The main areas for irrigation are the
north-west of the West Bank, in the vicinity of Jericho and the Upper Jordan Valley.
A significant amount of work has been undertaken on calculating recharge to the aquifers in
the West Bank and in the Western Aquifer Basin by measuring discharge and abstraction as a
surrogate for recharge. However, most of the estimates rely on empirical relationships
between annual rainfall and recharge. Estimates undertaken using an empirical method are
not physically based, but nonetheless can be used as a guide to determine whether the
recharge calculated by the modelling are realistic. The estimates for the Western Aquifer
Basin are around 350 Mm3 a-1 and 800 Mm3 a-1 for the West Bank as a whole.
To enable recharge to be calculated using a physical basis over aquifer outcrops, a distributed
recharge model has been developed and tested. An existing object-oriented groundwater flow
model has been adapted from an existing code. An object-oriented approach was chosen to
enable a range of recharge mechanisms to be incorporated easily into the model. Recharge is
calculated at a node, which is held on a grid and enables a distributed recharge estimate to be
undertaken. Four types of recharge node can be specified; soil moisture balance method,
wetting threshold, urban recharge process and irrigation losses. In addition to these
mechanisms, runoff routing to wadis and subsequent infiltration is implemented.
Wadi
Depression-focused recharge
Infiltration (HVAC)
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Depression-focused recharge
Infiltration (HVAC)
Groundwater discharge
Groundwater model
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MODFLOW
Infiltration (HVAC)
Groundwater model
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MODFLOW
Water balance
Groundwater model
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Depression-focused recharge
Drawdown (hydrology)
Aquifer test
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Depression-focused recharge
Infiltration (HVAC)
Water balance
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