Agronomic research

In terms of agronomic research, we selected six experiments that reported rates of change in the following proxy environmental indicators: soil erosion, soil moisture days, storm intensity, river discharge, soil fertility and the application of commercial and domestic manure fertilizers to boost crop production. Our re-interpretation of the data is to inform readers to view the outcomes from the opposing ecological models. We discuss the findings of these experiments in turn.

Soil erosion

Soil erosion control had remained a major preoccupation for agricultural development in the East African colonies. However, in our perusal of the articles published in the East African Agricultural and Forestry Journal, between 1930s and 1960s, we found less than 5 percent of the total number of articles reporting some aspects of soils research (Table 5.1). We found even fewer experiments that investigated soil erosion at the watershed scale.¹⁷

In a Tanzanian study, historical rates of soil erosion across different land- use systems were estimated at 200-600 m* km ² per year'¹.¹⁸ An aspect of soil erosion with which the colonial authorities were mostly concerned was the

Table 5.1 Rates of soil loss, soil moisture and soil fertility based on research studies conducted in Kenya and Tanganyika, 1950s-1960s

formation of gullies. The available opinion then is that the gullies—once formed—are difficult to stop.¹⁹ At the Mpwapwa research station²⁰ in central Tanganyika, the amounts of water run-off and sediments yield from experimental plots were measured over an eight-year period. The cultivated plots yielded an equivalent of 58.85 tons ha ¹ of sediments, compared to plots planted with bulrushes that yielded 40.35 tons hawhile the plots with grass cover yielded 22.28tonsha ¹ of sediments.²¹ The annual run-offs from the treatment plots—up-scaled at watershed scales—are summarized as percentages of runoff and soils lost (Figures 5.2A and 5.2B). The limitations of the scale notwithstanding, the important finding was that vegetation cover was necessary for soil conservation.

Figure 5.2 (A) Percentage run-off and (B) soil erosion in tons per acre per year, from treatment plots.

It should be noted that the experiment was conducted on a research station that did not reflect outcomes from a watershed or African farms. Thus, the results might be examined from two perspectives. The first is that data collected from micro-plots has limitations.²² At the watershed scale— which represents real land-use conditions, rainfall provides a coarse scale in which the runoff and sediment production reflect the status of land-use intensification.²⁵ Investigations at the scale of plots would not divulge effects of land-use intensification at scales of indigenous land use. Second, at the watershed scale, soils eroded from a site are not lost but are redistributed downstream.²⁴ The experiment may be compared with storm discharge and sediment production by flash floods.

Storm intensity and stream discharge

Sediment yields may be related to the amounts, duration and intensity of rainfall (Figure 5.3). Despite the limited data, the volume of water discharged at the watershed scale has a direct relationship with the volume of sediments in the stream discharge—which in turn might be influenced by the intensification of land use within the watershed.²’ From the relationships in the studies we identified (based on the limited data available), increase in rainfall resulted in corresponding increase in stream discharge. This relationship is an example of scenario A in Figure 5.1. Other factors such as existing soil moisture would also influence the volume of stream discharge.²⁶

The findings of various investigations²' show that vegetation cover plays an important role in influencing stream flows. The removal of vegetation, for whatever reason, is likely to increase the amounts of stream discharge, thus incurring potential soil loss. Large-scale land clearing is often carried

Figure 5.3 Relationship between storm intensity and discharge in an experimental watershed.

out by development projects but is not part of indigenous land-use practices (see Chapters 7 and 8). Considering that the East African region experiences erratic rainfall—varying between periods of drought and deluge—the amounts of sediment yield would also vary.²⁸ Investigations by other researchers²⁹ concluded that factors such as droughts and therefore moisture deficits would influence the effectiveness of rainfall in a watershed. Conversely, soil moisture days are crucial for crop production.