Spatio-temporal rainfall variation and stock management in arid Namibia

Abstract

The high inherent variability in rainfall and forage availability in arid regions makes it very difficult for a rancher to establish a herd size that is suited to the environment and also complicates management decision-making regarding the distribution of animals within and between areas of a single ranch. We recorded the temporal and spatial variability in rainfall and grass production on a local scale on 3 ranches in arid Namibia over 3 years, to determine their effects on potential stocking density, stock movement decisions and timing of grazing. We conducted a 3-year experiment to test the effects of the timing of grazing on grass regrowth. Furthermore, we tested the reliability of a rule-ofthumb used by ranchers to ascertain whether a drought year is likely and whether they should sell some of their cattle in advance of drought. We expanded the scale of the study to include 31 ranches over a 700 km-long rainfall gradient to determine the reliability of extrapolating conclusions obtained at a local scale to a larger geographic scale. We found that rainfall and grass production varied widely, resulting in recommended stocking densities up to 10-fold smaller than those currently recommended by the Namibian Ministry of Agriculture. We found that there is no optimal spatial or temporal scale of ranch management, and that ranchers could do no better than a simple rule of "move the cattle to wherever there is most grass ". This simple rule's validity was reinforced by the results of our experiment, which showed that there was no significant effect of timing of grazing on grass regrowth. Statistical analyses of long-term rainfall records indicated that the ranchers' rule-of-thumb regarding approaching drought was unreliable and that this could lead to substantial loss of profit if adhered to.

Livestock population dynamics and pastoral communities’ adaptation to rainfall variability in communal lands of Kgalagadi South, Botswana

“…This is consistent with a non-equilibrium ecosystem (Angassa and Oba 2007). In our study, livestock populations generally increased with increasing rainfall, which could be explained partially by the importance of rainfall on vegetation production (Ward et al 2004). In this study, the goat population was moderately associated with mean annual precipitation and basically decreased during drought years.…”

Multi‐scale patterns and bush encroachment in an arid savanna with a shallow soil layer

“…In the following, we summarize the hypotheses underlying our investigations. Based on previous observations that trees appeared to be distributed in patches consisting of one or two tree sizes, we hypothesized that tree sizefrequency distributions in small areas exhibit uni-and bimodality and are independent of average annual rainfall, within the limits set by our study area (80-170 mm/year: Ward et al 2004). At the landscape scale, we expected the tree size-frequency distribution to follow a negativeexponentially declining function as it would be caused by temporally continuous recruitment, continuous growth (resulting in a close size-age relationship), and a constant mortality rate (cf.…”

Impact of pastoral land use on central Namibian savanna vegetation - Methods for indictor development and perspectives for their application

“…Degradation in zentralnamibischen Weidegebieten äußert sich unter anderem im Verlust mehrjähriger Gräser, einer Zunahme an invasiven Arten, verstärkter Bodenerosion sowie Bodenverkrustung und der verringerten Wasseraufnahmefähigkeit des Bodens (Ward et al 2004;Klintenberg & Seely 2004;Getzin 2005). Vieles deutet darauf hin, dass trotz relativ konstant gebliebener Niederschläge die Produktivität der Weiden im Untersuchungsgebiet in den letzten 50 Jahren stark zurückgegangen ist .…”