3. Y ARRA RIVER CATCHMENT, DROUGHT HISTORY, AND DATA SOURCES AND PROCESSING
3.5. Hydro-meteorological Data Processing
3.5.4. Soil Moisture Data
Soil moisture content was used to account for the fluctuations in water stored in the plant root zone. As outlined in Section 3.4, the two-layer water budget model of Palmer (1965) was adapted in this study to calculate the monthly basin average soil moisture content. The model considers the soil to have two arbitrary layers (i.e., the surface layer and underlying layer) as shown in Figure 3.7.
Figure 3.7 Two layer water budget model for the Yarra River carchment (adapted from Palmer, 1965)
The upper layer (called surface layer) is roughly equivalent to the plough layer. This is the layer which receives rainfall and from which the evaporation takes place. In moisture accounting, it is assumed that evapotranspiration takes place at the potential rate from this surface layer until all available moisture in the layer is removed. Only then, moisture can be removed from the underlying layer of soil. Likewise, it is assumed that there is no recharge to the underlying layer until the surface layer soil moisture is brought to field capacity. The available water holding capacity of the soil in
the underlying layer depends on the depth of the effective root zone and the soil characteristics in the area under study.
Watson (1999) in a study of forest water yield in the Yarra River catchment used the maximum water holding capacity (both in the surface layer and underlying layer) as 670 mm (water) / 1000 mm (soil) under saturated conditions. This assumption was made following the analyses presented by Campbell (1999) using soil moisture tension data. Therefore, 670 mm (water) / 1000 mm (soil) under saturated conditions was used as the total available water holding capacity (AWC) in the two-layer water
budget model in this study. It should be noted that there are some non-forest area within the catchment; however, the selection of the above AWC value depends on soil
characteristics rather then whether forest or non-forest area. Similar to Palmer (1965), 25 mm was used as the water holding capacity for the surface layer and the remaining 645 mm was used for the underlying layer. Monthly water balance was then conducted in the two layers as was done by Palmer (1965), considering catchment average rainfall and potential evapotranspiration calculated in Section 3.5.1. In the water balance model, it was assumed that the surface runoff was generated only when available water was greater than the water holding capacity in both layers. Moreover, the loss from the underlying layer depends on initial moisture content (i.e., available moisture content at beginning of the month) as well as on the potential evapotranspiration (PE) and the AWC of the soil system. Therefore, the moisture losses from surface and underlying
layers were calculated using Equations (3.1) and (3.2) respectively.
Ls = Ss'or (PE−P) whichever is smaller, and (3.1)
Lu = ' ( ) u s S PE P L AWC − − , Lu ≤Su' (3.2)
where Ls = moisture loss from surface layer (mm), '
s
S = available moisture stored in surface layer at start of month (mm) (i.e., ≤ 25 mm),
PE = potential evapotranspiration for the month (mm), P = rainfall for the month (mm),
'
u
S = available moisture stored in underlying layer at start of month (mm)
(i.e. ≤ 645 mm), and
AWC = total available water holding capacity of the both surface and
underlying layer (i.e. 670 mm (water) / 1000 mm (soil) under saturated conditions).
The model was run with a start month (i) which considered the catchment soil
moisture content equal to the AWC. Therefore, the catchment soil moisture content at
the end of month i, Wi:
Wi = AWC= 750 mm (3.3)
For the following month i+1, the available moisture stored in the surface layer
at the start of month, ' ,
s i
S = 25 mm and underlying layer, Su i', = 645 mm. The
catchment soil moisture content at the end of month i+1, Wi+1, was then calculated as:
Wi+1 =
(
)
(
)
(
)
(
)
(
)
' ' , i , i ' ' , i , i ' ' , i , i , , , s u s u s u s u S S L L S S S S P PE ⎧ + − + ⎪⎪ + ⎨ ⎪ + + − ⎪⎩ when when when P P P PE PE PE > = < , Wi+1 ≤ AWC (3.4)Equation (3.4) implies that when PE is larger than P then there will be moisture
loss from the system, while when P is larger than PE then there will be moisture added
in the system until it reaches the AWC. If PE is equal to the P in any month then there
will not be any change in soil moisture content in the catchment for that month. The calculations were continued using Equations (3.1), (3.2) and (3.4) for the rest of the months.
The catchment average total yearly rainfall and potential evapotranspiration for the Yarra River catchment is shown in Figure 3.8. It shows that the year, 1960, was relatively wet and the rainfall was much higher than the potential evapotranspiration. Moreover, it was found that the rainfall was continuously much higher than the
potential evapotranspiration for six months from April to September in 1960. Therefore, for running the water budget model, the start month was considered as September 1960 and the AWC for this month was considered as 670 mm. The
catchment soil moisture content for the months following September 1960 were calculated using Equations (3.1), (3.2) and (3.4). As mentioned above, the months from April to September in 1960 were relatively wet, therefore the soil moisture content for the months from April to August in 1960 were also considered at AWC (i.e., 670 mm)
and back calculations were carried out to calculate the soil moisture content for the months from January to March in 1960.
Figure 3.8 Catchment average total yearly rainfall and potential evapotranspiration for the Yarra River catchment
3.6.
Summary
The Yarra River catchment located in Victoria (Australia) is a valuable asset to all Melbourne residents. The water resources from this catchment are important in terms of a wide range of water uses as well as downstream user requirements and environmental flows. Many initiatives have been taken by several water authorities including EPA Victoria and Melbourne Water Corporation for protecting catchment waterways and mitigating water demand in drought circumstances. However, frequent droughts and increasing water demand in recent years have increased pressure upon
water resources management within the catchment, and therefore, the management of water resources in terms of droughts is important within the Yarra River catchment.
Data on several hydro-meteorological variables (i.e., rainfall, potential evapotranspiration, streamflow and storage reservoir volume) were collected for the Yarra River catchment from Bureau of Meteorology, SILO database and Melbourne Water Corporation to compute the Drought Indices (DIs) for use in the catchment and for drought forecasting model development work. However, the soil moisture content data were not available for the catchment, and therefore the two-layer water budget model of Palmer (1965) was adapted to determine the soil moisture content in the catchment.
Data were collected or estimated for the period from 1960 to 2008 (49 years). These data were available in two time scales (i.e., daily and monthly) for these hydro- meteorological variables. However, DIs and drought forecasting model were developed using a monthly time step as the monthly drought forecasting was suitable for operational purposes, and also monthly data have lower sensitivity to observational errors. Therefore, the data that were not on monthly basis were converted to represent the monthly time scale. These data (i.e., rainfall, potential evapotranspiration, streamflow, storage reservoir volume and soil moisture content) were collected and/or estimated, and analyzed for three specific purposes in this thesis for use in the Yarra River catchment:
1. To evaluate the existing drought indices (Chapter 4),
2. To develop the Nonlinear Aggregated Drought Index (Chapter 5), and 3. To develop the drought forecasting model (Chapter 6).