Detecting the short term impact of soil and water conservation practices
from incomplete monitoring records - A case study from the Tana
sub-basin, Ethiopia
Abstract
Efforts to tackle land degradation worldwide have spurred the adoption
of soil and water conservation (SWC) practices intended to reduce
surface runoff and erosion. Despite their widespread implementation,
missing or incomplete monitoring remains a pervasive problem preventing
evaluation of how well SWC practices meet these aims. Key metrics to
evaluate SWC efficacy are the production of flow per unit rainfall
(runoff ratio), and exported sediment (sediment concentration). We
develop a method to assess changes in these metrics in the absence of a
flow rating curve, using more complete and reliable measurements of
stage (flow depth). We apply these methods to incomplete monitoring
datasets collected from five watersheds included in the Tana and Beles
Integrated Water Resource Development Project (TBIWRDP) in the Abay
(Blue Nile) basin, Ethiopia. Changes in runoff ratio and sediment
concentration relative to the first year of treatment varied by season.
In the long wet season (Kiremt) that generates most runoff and erosion,
reductions in runoff ratio occurred in three watersheds, and reductions
in sediment concentration in four watersheds. Reductions in the runoff
ratio were directly proportional to the areal density of SWC treatments
in the watersheds, suggesting that SWC treatments were effective in
controlling runoff and erosion. We suggest that stage and sediment
concentration information can be used to assess watershed responses to
SWC treatments. Focusing on these relatively robust measurements, may
facilitate the design of reliable and affordable monitoring programs,
and ultimately facilitate improved financing approaches based on
reasonable estimates of likely SWC practice performance.