Potential of Shallow Groundwater for Household Level Irrigation Practices in Tahtay Koraro Woreda, Tigray, Northern Ethiopia

This paper deals with the results of a pilot study conducted to estimate the shallow groundwater resource potential and irrigation capacity at the household level in Tahtay Koraro Woreda, northwestern zone of Tigray, Ethiopia. The potential evapotranspiration and actual evapotranspiration of the study area are estimated to be 1484 mm/year and 814 mm/year respectively. The runoff is approximately calculated to be 280 mm/year and the annual groundwater recharge is estimated to be 29 mm/year. The total annual groundwater abstraction for human, livestock, and irrigation is estimated to be 25 mm/year. It should be noted that the groundwater recharge rate is expected to remain constant while the total annual groundwater discharge is expected to increase from year to year. This relation when projected over a long period may result in a negative groundwater budget which can result in depletion of groundwater (lowering of groundwater levels), reduced baseflow to streams, and deterioration of water quality.  The computed values for hydraulic conductivity of the aquifers range from 1.63 m/day to 7.27 m/day with an average value of 4.9 m/day and transmissivity from 48.9 m2/day to 218.1 m2/day with an average value of 147.14 m2/day. The aquifers in the highly weathered basalt and highly weathered siltstone – sandstone intercalation have transmissivity values ranging from 99 m2/day to 218.1 m2/day with an average value of 157 m2/day and are grouped into the moderate potentiality aquifers category. The aquifers in the slightly weathered and fractured metavolcanics grouped under low potentiality based on the lower transmissivity values (<50 m2/day). The study area has low to moderate groundwater potentiality, hence, large-scale groundwater pumping is not possible. Therefore, the current activity of using hand dug wells for household-level irrigation is the best way of using groundwater for irrigation and other uses as well. Increasing the depth of the existing hand dug wells that are constructed in highly weathered basalt and highly weathered siltstone – sandstone intercalation can also enhance the yield of the hand dug wells. It is recommended to use water-saving irrigation technologies rather than increasing the number of wells. This will also help in increasing the irrigation area. Groundwater recharge enhancement structures such as trenches, percolation ponds, and check dams be constructed in scientifically selected localities to further enhance the groundwater potential

Evaluation of Groundwater Quality and Suitability for Drinking and Irrigation Purposes Using Hydrochemical Approach: The Case of Raya Valley, Northern Ethiopia

Groundwater is a vital source of water for domestic and agricultural activities in Raya valley due to non-perennial flow of surface water.Thus the present study aims to evaluate the groundwater quality and assess its suitability for drinking and irrigation purposes. A total of 137 groundwater samples were examined for various physico-chemical parameters to evaluate the quality and suitability of groundwater for the intended purposes. Piper-Hill diagram was adapted to infer hydro-geochemical facies and water types. The suitability of groundwater for drinking purpose was evaluated by comparing the analytical results of different water quality parameters to the Ethiopian standard as well as World Health Organization Standards (WHO, 2006). Salinity, sodicity and related parameters were considered to evaluate the suitability of the groundwater for irrigation use. The dominant water types are magnesium bicarbonate and sodium-calcium bicarbonate.The hydrochemistry of the groundwater changes from Mg-Ca-HCO3 dominance at the foot of volcanic hills to Ca-Mg-Na-HCO3 and Ca-Na-Mg-HCO3 at the valley floor. On the basis of total hardness, 46 % of the water samples in Raya valley fall in the very hard water class whereas, based on TDS values 95% of the water samples are found to be permissible for drinking. With the exception of three constituents (Mg2+, Ca2+ and K+), all other analyzed constituents in groundwater are within the established standards of Ethiopia. Water suitability for irrigation indicates that the groundwater fall in the range of good to permissible, however, higher salinity, magnesium hazard and low permeability index restricted its suitability in southeastern parts of the study area.Keywords: Water quality; SAR; Irrigation and drinking suitability; Raya Valley; Ethiopi

High-resolution Data Based Groundwater Recharge Estimations of Aynalem Well Field

Due to the ever-increasing demand for water in Aynalem catchment and its surrounding, there has been an increased pressure on the Aynalem well field putting the sustainability of water supply from the aquifer under continuous threat. Thus, it is vital to understand the water balance of the catchment to ensure sustainable utilization of the groundwater resource. This in turn requires proper quantification of the components of water balance among which recharge estimation is the most important. This paper estimates the groundwater recharge of the Aynalem catchment using high-resolution hydro-meteorological data. Daily precipitation and temperature measurement data for years 2001-2018; groundwater level fluctuation records collected at every 30 minutes; and soil and land use maps were used to make recharge estimations. In the groundwater level fluctuation, three boreholes were monitored, but only two were utilized for the analysis because the third was under operation and does not represent the natural hydrologic condition. Thornthwaite soil moisture balance and groundwater level fluctuation methods were applied to determine the groundwater recharge of the Aynalem catchment. Accordingly, the annual rate of groundwater recharge estimated based on the soil-water balance ranges between 7mm/year and 138.5 mm/year with the weighted average value of 89.04 mm/year. The weighted average value is considered to represent the catchment value because the diverse soil and land use/cover types respond differently to allow the precipitation to recharge the groundwater. On the other hand, the groundwater recharge estimated using the groundwater level fluctuation method showed yearly groundwater recharge of 91 to 93 mm/year. The similarity in the groundwater recharge result obtained from both methods strengthens the acceptability of the estimate. It also points out that the previously reported estimate is much lower (36 to 66 mm/year)

Evaluation of groundwater resources in the Geba basin, Ethiopia

This article presents an assessment of the groundwater resources in the Geba basin, Ethiopia. Hydrogeological characteristics are derived from a combination of GIS and field survey data. MODFLOW groundwater model in a PMWIN environment is used to simulate the movement and distribution of groundwater in the basin. Despite the limited data available, by simplifying the model as a single layered semi-confined groundwater system and by optimising the transmissivity of the different lithological units, a realistic description of the groundwater flow is obtained. It is concluded that 30,000 m(3)/day of groundwater can be abstracted from the Geba basin for irrigation in a sustainable way, in locations characterised by shallow groundwater in combination with aquitard-type lithological units

Groundwater Based Irrigation and Food Security in Raya-Kobo Valley, Northern Ethiopia

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

Groundwater exploitation: safe yield versus sustainable yield: example of the Aynalem well field, Mekelle

The concept of safe yield is often being used to refer to the amount of groundwater that can be exploited from a well or from an aquifer. Although many definitions have been given for this concept, since its introduction in 1915 by Lee, it could be defined as the quantity of groundwater that can be withdrawn on a permanent basis, whereby a new dynamic equilibrium is reached, and the aquifer storage has been brought to a new constant level, thus preventing the further decrease of hydraulic heads. This condition can only be reached by an increased recharge and a decreased discharge. Theis (1940) gave a name to the sum of both: the capture. In overexploited groundwater systems (beyond safe yield), also part of storage is captured, resulting in continuously lowering water levels. The safe yield is frequently estimated from the natural aquifer recharge, based on the reasoning that total recharge, or a fraction thereof, can be withdrawn without changing the system. This reasoning is completely false, and has been termed by Bredehoeft (1997; 2002): “The water budget myth”. Usually, for unconfined aquifers, recharge is assumed to be constant, as being delivered by precipitation (Bredehoeft, 1997) and the capture is equal to the decreased discharge. When starting up a new groundwater exploitation, water is mainly derived from storage. As evolution progresses towards a new equilibrium, the contribution of storage decreases, while capture increasingly delivers the pumped water. Ultimately, a new dynamic equilibrium is reached, with constant, but lowered groundwater levels. This decrease in discharge implies that springs, surface waters, … may have dried up. The time for this new equilibrium to establish (provided it is reached), may be long. Moreover, the localisation of the new well field matters for determining this dynamic response: the safe yield of an aquifer cannot be quantified without identifying concrete pumping stations. This important condition is usually neglected. This safe yield is not necessarily sustainable. The definition of the sustainable yield of an aquifer is not an exclusively hydrological problem. It needs to be based on the acceptability of a certain lowering of groundwater levels. In other words, different options are possible. Next to hydrological thoughts, also other environmental concerns (e.g. fauna and flora) have to be considered, and moreover social and economic factors are important. Sustainable groundwater development is a multi-dimensional problem, which cannot be solved by hydrogeologists alone. Groundwater models are used to study the dynamic response of groundwater systems to specific stresses, including groundwater exploitation. However, hydrogeologists need decisions about options, to be made by other stakeholders, as an input for their models. For example: is this certain spring important to society, for nature conservation, … or can it be sacrificed for the sake of an increased water supply from a well field?. Without pretending to solve the question of sustainability of the Aynalem well field, this example was studied by calculating natural groundwater recharge and by (an attempt to) quantify the drawdown around the well field using a mathematical model