Soils 99

The soil properties (Section 4.2.1) were then used by the GeoSFM to set the hydraulic parameters that govern the subsurface water movement and changes in soil moisture content over the Zambezi Basin. For soil depth characterisation, the Global Data Set of Soil Particle Size Properties produced by Webb et al. (1993) was used. The data set specifies the top and bottom depths and the percentage of the texture type (sand, silt, and clay) of soil horizons in 106 different soil types, catalogued for nine regions. Detailed procedures for extracting the required grids from these data sets have been described in USGS (2000a) and the procedures are performed outside of GeoSFM. Below is a description of the derivation processes for the parameters for the study area.

Soil Texture: The soil texture class was generated for each sub-basin using the GeoSFM and the FAO soil data attributes were used as input. The FAO soil data attributes contain a record number, and the percentages mapping unit for each sub-basin (Table 4.2 Section 4.2.1). The criteria defined in USGS (2000a) and USGS (2002) to identify the soil texture type are:

 “1" sandy (coarse) soils;  "2" loamy (medium) soils, and  "3" clay (heavy) soils.

The governing equation for soil texture calculation for each sub-basin (as described in USGS (2000a) and Entenmanns (2005)) was based on the soil data attributes and one assumption. For example: if it was assumed that a mapping unit "A" in the Western and Southern parts of the Zambezi River Basin was 95% coarse, 4 % medium, and 1% heavy then the calculation

for the estimated soil texture was:

Estimated Soil Texture = ((95/100) * 1) + (4/100) * 2) + (1/100) * 3) Estimated Soil Texture = 0.95 + 0.08 + 0.03

Estimated Soil Texture = 1.06 Estimated Soil Texture = 1

 In the Northern and Eastern parts of the basin the mapping unit was based on 40% coarse, 20% medium and 20% heavy giving a soil texture index of 2.0.

 For the wetland areas the mapping unit was calculated as 1% coarse, 4% medium, and 95% heavy soil texture giving a soil texture index of 2.85.

The soil texture data (Figure 4.7a) was used in the GeoSFM model as an input value to estimate the hydraulic conductivity – which plays an important role in the estimation of soil water holding capacity – by accounting for the amount of water being retained by different soils texture types according to their infiltration rate capacity over the Zambezi Basin. The texture is also used in conjunction with land cover type to generate the SCS Runoff Curve Number.

Hydraulic conductivity: The empirical approach (similar to the one used for the calculation

of soil texture described above and in USGS (2000a)) was used by the GeoSFM model to estimate the spatial distribution of saturated hydraulic conductivity in the Zambezi Basin. The approach estimates Ks by correlating the various soil properties (such as porosity and matrix potential distribution and soil texture). The range of values of soil properties used to estimate the Ks for each sub-basin within the Zambezi Basin are described in Table 4.2, Section 4.2.1. The Ks was calculated for each region (Figure 4.7b) as follows:

 In the Western and Southern parts of the Zambezi River Basin the mapping unit "A” in the FAO soil parameters was 95% coarse (sand), 4% medium (loamy), and 1%

heavy (clay) then the calculation for the estimated hydraulic conductivity was: Estimated hydraulic conductivity = (95/100) * 1) + ((4/100) * 0.01) + ((1/100) * 0.001) Estimated hydraulic conductivity = (0.95 * 1) + (0.04 * 0.01) + (0.01*0.001)

Estimated hydraulic conductivity = (0. 0.95) + (0. 0.0004) + (0.00001) Estimated hydraulic conductivity = 0.95 hr-1

 In the Northern, Central and Eastern parts of the basin the mapping unit “A” was based on 40% coarse, 40% medium and 20% heavy giving an average estimated sutured hydraulic conductivity of 0.44 cm hr-1_.

a)

b)

Figure 4.7: Spatial distribution of soil texture class (a) and saturated hydraulic conductivity (b) in the Zambezi Basin (FAO, 1995)

Soil depth: The soil depth data are used by the GeoSFM model, together with the daily

available soil water (estimated by the daily water balance and Ks) to estimate the rate of percolation to deep groundwater (USGS, 2000a; Entenman, 2005). The soil depth classes were calculated for each region Figure 4.8a)as follows:

 In the Western and Southern part of the basin the spatial distribution of the soil depth (Unit “A”) was calculated as 50% shallow, 60% moderately deep, 20% deep, and 15% very deep, giving an average soil depth of 101.5 cm;

 In the North-East and certain areas of Central of the basin, the spatial distribution of soil depth (Unit “A”) was calculated as 5% moderately deep, 15% deep, and 80% very deep, giving an average soil depth of 182.5 cm;

 For the large areas of Central and South-East of the basin, the spatial distribution of soil depth (Unit “A”) was calculated as 10% very shallow, 50% shallow, 10% moderately deep, 10% deep, and 20% very deep, giving an average soil depth of 75.5 cm.

Soil Water Holding Capacity (SoilWHC): The GeoSFM model used the soil depth values, in

conjunction with soil texture data, to determine the average soil water holding capacity (SoilWHC) in each of the sub-basins within the Zambezi Basin. Based on FAO data for the Zambezi Basin, the GeoSFM classified SoilWHC into four hydrologic soil groups (Figure 4.8b) according to their soil water holding capacity as follows:

 Group “A” consists of soils that have a high soil moisture capacity, low runoff potential and high infiltration rates. The soil textures included in this group are sand, loamy sand, and sandy loam. This group of soils is mainly predominant in the North and Central areas of the Zambezi Basin;

 Group “B” consists of soils that have moderate moisture capacity and infiltration rates. The soil textures included in this group are silt loam, and loam. This group of soils occurs in the Western, some areas of the Central, East and South East regions of the basin;

 Group “C” consists of soils that have low moisture holding capacity. The only soil texture included in this group is sandy clay loam. Spatially, this soil occurs in the West, Central and North-East of the Basin;

 Group “D” consists of soils that have very low moisture holding capacity. The soil textures included in this group are clay loam, silty clay loam, sandy clay, silty clay, and clay. These soils predominantly occur in the West and North-West of the basin. Based on the above assessment it was assumed that, in the North-East and small areas of the Central part of the Zambezi Basin, the predominant hydrological soils are very deep with high infiltration rates; in the West and South and large areas of the Central areas and along the river banks, the predominant soils are deep and moderately deep with high to moderate infiltration rates (Figure 4.8).

a)

b)

Figure 4.8: Spatial distribution of soil depth classes (a) soil depth in (cm) and (b) soil water holding capacity in (mm m-1_{) for the Zambezi Basin (Source: FAO, 1995)}