Rainfall and Land Degradation

Abstract

The complexity of the notion ‘land’ and its scale features leads to many different definitions of land and land degradation. Among the components of land degradation are desertification, soil degradation and erosion. There is a wealth of literature claiming that land degradation is serious. These so-called doom papers are based on ‘hard’ facts from remote sensing, computer models and measurements. However, other papers have raised the question ‘how serious is land degradation?’ In any discussion of land degradation there are four spatial-temporal scales that should be distinguished: regional, watershed, field and point. At each scale, one may use different proxies for land degradation. When assessing land degradation, ‘experts’ tend to overestimate the phenomenon; they should take better account of its spatial and temporal dimensions. One of the main reasons they currently overestimate land degradation is that they underestimate the abilities of local farmers, many of whom have been able to modify their land management. In order to study land degradation at multiple scales it is also necessary to study rainfall at multiple scales. Rainfall can be analysed for land degradation at four different scales: from the ‘small’ annual scale to the ‘large’ minute scale. Besides scales one may also distinguish between average values and temporal and spatial variations. In this paper, one or more examples of rainfall data are presented for each scale and interpreted with respect to land degradation. At the annual scale, trend analysis and rainfall probabilities are important. The decadal (10-day) scale is especially suitable for calculating the varying lengths of the growing season. At the scale of one day, the size classes of showers, return period (design storm), hydrological and agronomic modelling and dry spell analysis are discussed. At the minute scale, erosivity in El Niño and La Niña years is important.

Rainfall meets land at the soil surface. Rainfall is divided over several pedohydrological components. Green water is that part of rainfall that is stored in the soil and available to plants. Land degradation decreases infiltration, waterholding capacity and transpiration, but enhances runoff and soil evaporation. These agrophysical processes decrease the Green Water Use Efficiency (GWUE; the ratio of transpiration to precipitation). Special attention is given to estimating the effects of land degradation on ‘computing available soil moisture’ in order to understand what farmers perceive as drought. Rain falling on the land may be intercepted by vegetation, run off the ground surface, or infiltrate into the soil; this is reflected in the rainwater balance. Infiltrating water may be stored in the root zone or drain below the root zone to groundwater and stream base flow, contributing what is nowadays called ‘blue water’. These processes are reflected in the infiltration water balance. The maximum amount of water stored in the root zone available for plant growth is a very important soil characteristic because it determines the potential survival of plants in a dry spell. Water stored in the root zone may be lost as evaporation from the soil surface into the atmosphere, or taken up by plants and lost as transpiration. This is reflected in the soil water balance. In drylands in sub-Saharan Africa the GWUE ranges from 5–15%, which is very low. In East Africa it may reach 20%, but in comparable climates in the USA the GWUE may be above 50%.

The concepts of land degradation mitigation are derived from the rainwater balance. After drawing a number of conclusions, it is suggested how we could improve our understanding of land degradation by improving the availability of rainfall data at multiple scales.