Two rapid appraisals of FAO-56 crop coefficients for semiarid natural vegetation of the northern Ethiopian highlands

In the highlands of northern Ethiopia, degraded hillslopes are closed for grazing and agriculture for environmental rehabilitation. In order to assess the effect of these exclosures on water balances, a correct assessment of evapotranspiration is needed. A common method for calculating evapotranspiration employs the reference evapotranspiration in combination with a crop coefficient. However, for natural vegetation, these crop coefficients are not readily available. Therefore, the objective of this study was to derive crop coefficients for semiarid natural vegetation using two different approaches. The crop coefficients were assessed with the dual crop coefficient approach as outlined in the FAO Irrigation and Drainage Paper n°56 and compared with crop coefficients derived from a simple soil water balance. A good agreement between both methods was found. The mid-season crop coefficients ranged from 0.50 to 0.85 for open vegetation with small shrubs and from 0.85 to 0.95 for well developed shrubland. Vegetation with a closed tree canopy was characterized by a mid-season crop coefficient between 0.95 and 1.1. Additionally, a significant logarithmic relation between vegetative soil cover and crop coefficient was found. Using this relation, crop coefficients can be easily derived for semiarid shrub savannahs in similar environments and used for the calculation of evapotranspiration.     

Frankincense yield assessment and modeling in closed and grazed Boswellia papyrifera woodlands of Tigray, Northern Ethiopia

Boswellia papyrifera, a frankincense producing tree, grows in the arid lowlands of Ethiopia. It is a multipurpose tree species with ecological, environmental, cultural and socio-economic values. The resource has been declining due to unsustainable management. This study is aimed at estimating frankincense yield in a single production year, assessing the relationship between yield and dendrometric variables, and developing predictive yield models. We collected data of one harvesting season from randomly selected sample trees in an exclosure and a free grazing site. We found strong nonlinear relations between yield and dendrometric variables, which are useful for predictive yield modeling. A stepwise linear regression indicated that the yield from the sixth round of tapping could explain about 88% of the variation in annual yield per tree, a finding that can largely facilitate future yield monitoring. The frankincense yield was larger in the exclosure than in the adjacent free grazing site and the difference was statistically significant. To conclude, management of degraded B. papyrifera forest as exclosures should be strengthened to enhance the economic, environmental and cultural benefits from the species. Their effectiveness should be evaluated through yield monitoring and prediction.