This study evaluates the cause of salinization in an irrigation scheme of 100 ha supplied from a reservoir. The scheme is located in Gumselasa catchment (28 km2), Tigray region, northern Ethiopia. The catchment is underlain by limestone–shale–marl intercalations with dolerite intrusion and some recent sediments. Water balance computation, hydrochemical analyses and irrigation water quality analyses methods were used in this investigation. Surface waters (river and reservoir) and groundwater samples were collected and analyzed. The water table in the irrigated land is ranging 0.2–2 m below the ground level. The majority of groundwater in the effective watershed area and the river and dam waters are fresh and alkaline whereas in the command area the groundwater is dominantly brackish and alkaline. The main hydrochemical facies in the groundwater in the effective watershed area are Ca-Na-SO4-HCO3, Ca-Na- HCO3-SO4, and Ca-Na-Mg-SO4-HCO3. The river and dam waters are Mg-Na-HCO3-SO4 and HCO3-SO4-Cl types, respectively. In the command area the main hydrochemical facies in the groundwater are Ca-Na-HCO3-SO4 and Ca-Na-Mg-SO4-HCO3. Irrigation water quality analyses revealed that salinity and toxicity hazards increase from the effective watershed to the irrigated land following the direction of the water flow. The results also showed that the analyzed waters for irrigation purpose had no sodicity hazard. The major composition controlling mechanisms in the groundwater chemistry was identified as the dissolution of carbonate minerals, silicate weathering, and cation exchange. One of the impacts of the construction of the dam in the hydrologic environment of the catchment is on its groundwater potential. The dam is indirectly recharging the aquifers and enhances the groundwater potential of the area. This increment of availability of groundwater enhanced dissolution of carbonate minerals (calcite, dolomite, and gypsum), silicate weathering and cation exchange processes, which are the main causes of salinity in the irrigated land. The rising of the brackish groundwater combined with insufficient leaching contributed to secondary salinization development in the irrigated land. Installation of surface and subsurface drainage systems and planting salt tolerant (salt loving) plants are recommended to minimize the risk of salinization and salt accumulation in the soils of the irrigated land.

This manuscript is an attempt to demonstrate effectiveness of nature-based solutions (NBS) and measures to reduce risk of flooding and environmental impact in urban settings. The nature-based solutions (NBS) were assessed as scenarios from experience of urban storm drainage and sewerage systems based on practices that improve urban water management through modelling using urban stormwater management model (SWMM). The model has been applied in a typical urban environment in the second city in Botswana, the City of Francistown, which has a population of more than one hundred thousand. By considering the 2-yr and 10-year storm events in a calibrated SWMM, NBS scenarios from a mix of low impact and drainage measures were considered. The considered NBS scenarios were used to determine their effectiveness in terms of reducing and controlling peak runoff, flood volumes, infiltration and evapotranspiration in the study area, which are vital in assessing the opportunity and challenge for sustainable management of water resources and associated tradeoff of investments in the urban contexts. The study demonstrates the usefulness of implementing effective measures for achieving NBS in urban context and possibility of outscaling at basin and regional levels.

No abstract

The Raya-Kobo valley is characterized by good groundwater resources potential (excess of 198 Mm3) located in northern Ethiopia. The total area of the valley is approximated to be 3510 Km2. Despite this potential of groundwater, the area has been suffering from droughts. Establishment and expansion of cooperative societies, small business enterprises and irrigation farming were given priority as interventions to address the food security problem in the valley. These interventions were designed and implemented in the last several years by the regional governments administrating the Raya-Kobo valley, and groundwater is used as a source of water for irrigation farming for more than nine to ten years. Nevertheless, food insecurity is still a major problem in the area. The present study has been conducted to investigate the extent of contribution of the groundwater based intervention in the elimination food insecurity. The study was conducted by collecting, analyzing and interpreting all the existing relevant data of the study area. Groundwater potential was compiled from the author"s previous works and recently conducted research works. The impact of the groundwater based irrigation on food security issues was assessed using informal and formal interviews and focus group discussions. Limited area coverage of the irrigated land and the long time taken for the extension works in those drilled productive wells are the major limitations in the Kobo valley that makes the groundwater based irrigation unable to release the farmers of the area from the food insecurity. In Raya valley, besides to these problems, absence of responsible office, failure of pump, and the long time taken for pump maintenance are also the major factors. In Raya-Kobo valley food security is highly related to the availability of water. Irrigating the whole potential irrigable area, increasing and sustaining availability of groundwater and management of the irrigation scheme are vital to eradicate food insecurity from Raya-Kobo valley.

The paper presents results related to water balance model of the Gumselassa catchment (28.1 km2), Tigray, Northern Ethiopia. The catchment includes a small dam called Gumselassa dam having an effective watershed area of 22.14 km2 with reservoir capacity of 1.92 x 106 m3 and command area of about 1sq. km. The hydrology of the area was characterized on the basis of land use, soil, slope and climatic parameters. Different methods were employed in this study: rainfall coefficient method was used to determine monthly distribution of rainfall; Penman method to calculate evaporation from the reservoir; Thornthwaite method and Thornthwaite water balance model to determine potential and actual evapotranspiration; runoff coefficient method to estimate runoff; and, the water balance model was used to quantify the recharge. The catchment is characterized by one rainy season (three months) and two dry seasons (nine months) during the year. The mean annual rainfall of the catchment is 485.89 mm. The total annual water loss by evaporation from the reservoir is 1263.27 mm. The mean annual actual evapotranspiration of the effective watershed area one and two is 318.57 mm and 310.27 mm, respectively. The mean annual actual evapotranspiration of the water contributing area 1 and 2 to the command area is 337.06 mm and 355.29 mm, respectively. The mean annual actual evapotranspiration of the command area is 319.3 mm. The mean annual runoff generated from the effective watershed area one and two is 1.167 and 0.44 million cubic meters, respectively. The mean annual runoff generated from the water contributing area 1 and 2 to the command area is 0.048 and 0.349 million cubic meters, respectively. The mean annual runoff generated from the command area is 0.0875 million cubic meters. The total amount of water which is actually available to recharge the groundwater within the catchment is 4.065 million cubic meters, and any application of water for irrigation from the reservoir should take into account this readily available water.