Towns Landscape

The most important hydrological processes in the town landscape as described in section 4.1 are runoff, evapotranspiration, groundwater outflow and abstraction. The difference between town I and II landscapes is the soil depth and rate of abstraction (Table 5.3). In town I, the soil depth is moderate (3 m) and the rate of abstraction is set at 0.05 mm/d. An examination of the detailed WaSim output shows that the water table is absent in the dry season which represent a realistic condition in the area, because the dug wells became dry during some part of the year. In town II however, the soil is deep (10 m) and abstraction is put at 0.1 mm/d due to the presence of water table throughout the year in the dug wells.

For the town landscapes, the results are presented in Table 5.6 (a & b) and 3 set of graphs Figure 5.2 - Figure 5.4)

Figure 5.2 - Figure 5.4 presents the graphs in different colours such as daily rainfall (blue), runoff (red), AET (green), ETo (light blue), groundwater outflow (purple) and water table depth (orange) for the town landscapes during the wet and dry years. The same colours will be used in all similar graphs presented in this chapter.

The major observations in town landscapes are:

 There is little difference in the water balance results between the two town landscapes for the wet years (Figure 5.2a & Figure 5.3a) and for the dry years (Figure 5.2b & Figure 5.3b); most of the following discussion is therefore, applicable to both landscapes.

 The water loss through runoff for both town landscapes I and II during the three wet years is about 33 % of the rainfall (Table 5.6b).

 In the three dry years, the water loss due to runoff is only 18 % in the two town landscapes (Table 5.6b) occurring mostly when there are high rainfall events.

 Apart from 6^th September 2003, runoff is below 20 mm/d throughout the dry years; see Figure 5.2 & Figure5.3b. However, in the wet years there are many days when runoff is over 20 mm/d, see Figure 5.2 & Figure 5.3a.

 The AET is even higher than runoff in the wet years equalling about 49 % of the rainfall.

 The AET in dry years is however higher than runoff (Table 5.6a), taking about 80 % of total rainfall.

 In Figure 5.2 (a), runoff occurs as a consequence of each rainfall events in the wet years, but when there are 7 or more days without rainfall, very small or no runoff is produced when rainfall resumes.

 The low infiltration of the shallow soil zone on this landscape leads to substantial runoff generated during periods of intense rainfall or in peak rainy season when the soil moisture is high and the water table is close to the surface (Figure 5.4).

 The groundwater outflow indicated in Figure 5.2 & Figure 5.3 for the three wet years, for both shallow and deep weathered zones also have similar results, representing about 20 % and 19 % respectively. The outflow in dry years is however very low, having just about 2. % each for the shallow and deep zones (Table 5.6b).

Figure 5.2: Daily rainfall, runoff, AET and ETo for Town I LUs in the wet and low rainfall years

In Figure 5.2 (a & b), any rainfall above 30 mm/d produces simulated runoff even in the early rainy season. This is consistent with the nature of the surfaces in town landscape (such as roofs, tarred and paved surfaces) represented in the model set up by the curve number which produces much runoff due to limited infiltration.

In dry years however, runoff also occurs typically after any rainfall above 30 mm/day.

In the middle and towards the end of the rainy season when the soil moisture deficit is very low, rainfall events below 10 mm/d produce some runoff as seen in Figure 5.3 (a &

b). This rarely occurs in the early rainy season due to high soil moisture deficit.

Figure 5.3: Daily rainfall, runoff, AET and ETo for Town II LUs in the wet and low rainfall years

The runoff in the deep weathered zone in all the Figure 5.2 & Figure 5.3 presented in this section slightly differs to the shallow weathered areas as seen in the results of Table 5.6. The runoff for the two town surfaces in WaSim is calculated using the SCS curve number method and the antecedent soil moisture condition. Because they have the same surface characteristics, this result in similar amounts of runoff generated.

Both the shallow and deep weathered zones have the same type of vegetation (small grasses) with the same percentage surface cover. The maximum root depth for the grass is 1.0 m; meaning that the rate of water extraction is the same for the shallow and deep

m for the deep weathered zones (Table 5.3) which did not exceed the maximum root depth in all the landscapes. This also resulted in similar amount of AET for the two town landscape areas.

Figure 5.4: Daily rainfall and water table depth for Town I landscape in the wet and low rainfall years

The difference in the weathered zone depths and height of the water table may also be the reason for the slight variations in the groundwater outflow for both the shallow and deep weathered areas as shown in Table 5.6a. The WaSim model only gives a response of an average water table decline and therefore cannot represent the cone of depression for an individual pumped well.