Cultivated Area Landscapes

The hydrological processes in the cultivated landscapes are highly influenced by anthropogenic activities. Changes in land use affect the physical, chemical and biological characteristics of soil (Shukla et al., 2003). Buytaert et al. (2006) found that the soil storage capacity of water in cultivated lands increases by 5 to 30%, and the hydraulic conductivity by 31% compared to non-cultivated lands. Buytaert et al. (2002) also found that the water retention of paramo soil at wilting point is reduced by 35 % due to cultivation which is significantly lower than the natural paramo soils not under cultivation.

Numerous studies have shown that the infiltration capacity of soils can be a good indicator of soil quality and health (Dexter, 2004, Shukla et al., 2006, Shougrakpam et al., 2010, Haghighi et al., 2011, Nyberg et al., 2012). Infiltration capacity is influenced by soil structure, aggregate stability, particle size distribution, land use type (Fu et al., 2003), vegetation, topographic and climatic influences (Jimenez et al., 2006). Land use change from natural vegetation to continuous cultivation and grazing has an adverse effect on soil bulk density, porosity, aeration, infiltration, storage, water transport characteristics and runoff. Most forest soils in semi-arid regions have the ability to absorb water at rapid rates, however, Cerda (1997) observed that cultivation increases soil surface storage by making a rougher (more bumpy) soil surface which increases infiltration of rainfall and as a consequence, the runoff and erosion rate is greatly reduced. This occurs not only in arid or semi-arid areas, but is also common in humid environments, e.g. Van der Kamp et al (2003) observed that an increase in the extent of cultivated land areas results in decreased discharges in the spring and autumn flood periods and a decreasing trend in the annual maximum peak discharge in Saskatchewan, Canada.

Some researchers have argued however, that cultivation decreases soil infiltration. Burt and Slattery (2006) for example argued that undisturbed soils have a much higher infiltration capacity than soils under cultivation due to compaction of cultivated soils.

The compacted soil will have a significantly higher bulk density and consequently become preferential zones of runoff generation. Though Cornelissen et al. (2013)

surfaces but not in catchments with savannah-type vegetation. Small animal’s burrows could act as efficient water infiltration galleries which can affect the hydrological characteristics of soils. Pasitschiniak-Arts and Messier (1998) showed that the abundance of small mammal burrows and ‘mole heaps’ in the dense nesting cover increases soil infiltration compared to where there is very little activity in the cultivated fields. Giertz et al. (2005) also demonstrates that reduced activity of the soil fauna on cultivated sites leads to a reduction of macropores in the soil and reduces saturated conductivity for cultivated plots compared to the plots with natural vegetation.

The hydrological processes of the cultivated lands in the study area also vary from one area to another depending on the type of soil described in chapter two. The different hydrological behaviours are described in section 4.2.1.

4.2.1. The flow processes in the cultivated landscape within the catchment

The cultivated landscapes in the study area cover the largest land area (30 %) of the catchment and are sub-divided into four groups based on the estimated depth of the weathered material, obtained from literature (e.g. Anderson and Ogilbee, 1973; Oteze, 1979; Kogbe, 1989; JICA 1990) and physical observation during the fieldwork. The first category is the two shallow weathered zones (3-6 m deep), with one shallow zone LU receiving additional run-on from exposed non- fractured hard rocks, while the second shallow zone LU doesn’t receive any additional run-on. The second categories are the deep weathered zone (6-10 m deep), also divided into two, with one receiving additional run-on while the second one doesn’t. Figure 4.4 illustrate the flow processes in the different cultivated landscapes.

Generally, the cultivated landscapes have input of precipitation and output of evapotranspiration and runoff. The soils of this landscape are loose at the surface due to cultivation and where the depth is sufficient, the soil allow more infiltration and moisture retention compared to town landscape units. The shallow weathered zones without run-on (Figure 4.4a) do not have sufficient depth to hold all infiltrated water, so that there is a tendency for saturation conditions to occur due to low storage. The

have more water due to saturation and even ponding on the surface. Water loss from this second LU with additional run-on and ponding is expected to be dominated by open water evaporation instead of evapotranspiration. The deep weathered zone on the other hand, have water table either at the lower level where there is no run-on input (Figure 4.4c), or at higher level close to the surface in areas that receive additional run-on input but the depth of the soil will not allow surface ponding as a consequence of saturation (Figure 4.4d). Groundwater in all the cultivated landscapes is lost through flow to streams (drains) or to the main river in the Fadama.

Figure 4.4: Schematic diagram showing the dominant flow processes during the wet season on (a) shallow weathered cultivated lands without additional run-on; (b) shallow weathered cultivated lands with additional run-on; (c) deep weathered cultivated lands without additional run-on; and (d) deep weathered cultivated lands with additional run-on

The researcher observed that each of the different LUs support different types of crops (Figure 4.5). Rice and maize for example are usually grown on either the shallow or deep soil cultivated lands receiving additional run-on due to their high water requirement (Figure 4.5 b & d), while Guinea Corn and Millet are grown on the shallow or deep soil cultivated lands without run-on (Figure 4.5 a & c) due to their low water requirement.

The crops will also have different seasonal cycles of evapotranspiration due to their

to vegetation differences. Evapotranspiration is higher when the crops are between the mid-stage (Figure 4.5 b & d) and maturity stage (Figure 4.5c) compared to the early stage (Figure 4.5a) of their growth because of larger crop cover and longer roots which usually coincides with the timing of the peak rainfall periods in the rainy season.

Figure 4.5: Pictures showing (a) Guinea Corn on shallow soil without run-on (early growth stage);

(b) Maize and Rice on shallow soil with run-on (mid-growth stage); (c) Millet on deep soil without run-on (maturity stage); (d) Rice on deep soil with run-on crop (mid-growth stage) during the wet season