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Study region: Zamra catchment, Upper Tekeze Basin, Northern Ethiopia. Study focus: Direct field estimates of potential evapotranspiration (PET) remain a challenge, because they are expensive and have limited spatial representativeness. An alternative is to derive PET by remote sensing as a product of reference evapotranspiration (ETo) and land use-land cover (LULC) factor (Kc). The aims of this study were to: i) validate satellite-derived Daily Reference Evapotranspiration (DMETREF-ETos); ii) correct the advection-bias of DMETREF-ETos as compared to in-situ FAO Penman-Monteith ETo (FAO-ETog); and iii) convert the bias-corrected DMETREF-ETos into PET. Four ATMOS 41 weather stations were installed and 1-year daily data was used to validate the DMETREF-ETos. The wet season actual evapotranspiration from FAO Water Productivity Open-access portal (WaPOR) was used to validate PET. New hydrological insights for the region: Evaluation of DMETREF-ETos showed large underestimation biases when compared to FAO-ETog derived from in-situ weather stations, particularly in warm conditions, due to unaccounted advection-bias. The bias correction was performed as a function of near-surface air temperature obtained from ERA5-Land, merged with in-situ air temperature; the results showed substantial improvement of ETo. Finally, the daily PET was estimated by multiplying the corrected DMETREF-ETos by a spatio-temporally variable Kc, defined as linearly dependent on the NDVI obtained from Sentinel 2. This study emphasized substantial difference between ETo and PET, implying that the ETo cannot substitute PET, which in many hydrological studies is still a common practice.

Exclosures are established with the objective of rehabilitating degraded lands and restoring of woody vegetation. Various studies have been conducted to evaluate the success of exclosure on restoring woody species diversity. However, works focusing on the effect of topographic factors on woody species diversity are scarce. Understanding the factors that determine woody species diversity is important for management purposes. Therefore, this paper analyzes the effect of altitude, slope, and aspect as topographic variables on woody species diversity in Dawsura exclosure in northern Ethiopia. Data on species identity, abundance, slope, elevation and aspect were recorded from 58 sampling plots. Different diversity indices were used to analyze the data and one-way ANOVA and linear regression was conducted. There were a total of 34 woody species represented 15 families, of which 62% and 38% were trees and shrubs respectively. Altitude (r2 = 0.63, p = 0.000 and r = 0.794, p < 0.01) and slope (r2 = 0.57, p = 0.002 and r = 0.68, p < 0.01) correlated significantly and positively with Shannon diversity, whereas aspect (r2 = 0.12, p = 0.378 and r = 0.27, p > 0.05) did not correlate significantly with Shannon diversity. Woody species diversity at moderate (1.44) and high (1.85) altitudes was significantly different from that of low (0.86) altitude areas (p = 0.0013). Furthermore, significantly higher woody species diversity was recorded at steep slope (1.88) and moderately steep slope (1.62) areas as compared to the gentle slope (0.95) areas. No significant variation was observed in woody species diversity among the aspect categories (p > 0.05). The study concludes that woody species diversity is largely regulated by slope and altitude than aspect in the exclosure. We suggest other environmental and anthropogenic variables should be taken into consideration in future studies on woody species diversity., Environmental science; Ecology; Environmental assessment; Environmental health; Environmental management; Environmental pollution; Exclosure; Topographic variables; Diversity; Woody species

Aridity is a risk with the threat of desertification because of erratic precipitation and prolonged droughts. Aridity indices are often used for the climate-based land classification and monitoring droughts. Hence, this study aimed at analysis of the spatial distribution of aridity indices in Raya valley, northern Ethiopia. Meteorological data from eight meteorological stations located throughout the study area were used to compute the three aridity indices employed: the De Martonne aridity index (I DM), the Pinna combinative index (I P), and the Food and Agriculture Organization aridity index (I FAO). Each aridity index value was computed at each station. Then, once the statistical properties of each aridity index at each station were assessed, inverse distance-weighted geographic information system interpolation technique was used for the spatial estimation of the aridity indices. The three indices indicated a high coefficient of determination which in common revealed the area prone to dry and semi-dry risk. However, I DM was found more appropriate for the considered area since it defines more the climate of each location using seven classes, and it can be computed at specific season and month. The annual, seasonal, and monthly aridity indices of I DM showed high spatial variability of aridity index. The spatial distribution of the I DM underlined irrigation requirements in the northern east part of the study area. This analysis may be helpful for the utilization of water resources and irrigation systems of the study area.