The Positive Effect of Micro-Dams for Groundwater Enhancement: a Case Study around Tsinkanet and Rubafeleg Area, Tigray, Northern Ethiopia)

The government of Tigray Regional State, Northern Ethiopia has been conducting a massive construction of micro-dams (small reservoirs) in order to decrease the rainfall dependency and alleviate food insecurity in drought prone areas of the region. Tsenkanet and Rubafeleg reservoirs are examples of this endeavor. The purpose of this investigation is to evaluate the role of these micro-dams in enhancing the surrounding groundwater by artificial recharge. In this study the physical characteristics of the reservoir are discussed including the groundwater surface water relationship with respect to water level elevation and also hydro-chemical composition. Six water samples have been analyzed. Samples are taken after rinsing the plastic bottles with distilled water and the sample to be taken. One surface water sample from each reservoir and one groundwater samples from each well next to each reservoir has been collected. Moreover, one sample from spring and one from river has been analyzed. The chemical composition of the reservoir water and the surrounding groundwater seem to suggest a similar origin and to be resulting from similar hydro-chemical processes. The chemical composition of all samples is found to be of the CaHCO3 type, which could be explained as resulting from precipitation water in which the mineral calcite has been dissolved. Both the topographic evidence and the water level monitoring data has confirmed the feeding of the reservoir to the near by shallow groundwater system. Quantifying the amount of groundwater accretion by the use of modeling and water balance method is recommended.


INTRODUCTION
Water scarcity and unwise management is a global concern especially in sub-Saharan countries. Global fresh water demand is alarmingly increasing with increase in population and civilization. As industrial, agricultural and domestic pollution threaten existing supplies -water becomes increasingly precious resource. © Mekelle University ISSN: 2073-073X Ethiopia surface water and groundwater resources have been regarded as high giving a name to the country as the water tower of east Africa (Said, 1993). This is factually true when considering half of the country, particularly the western and south western part of the country. The endowment can be used for productive purpose that can transform the countries socio economy (Selashi, 2007). Unfortunately its uneven distribution in space and time coupled with poor management and development of the resource lead the country to a repeated famine resulting from drought (FAO, 2005). The Tigray Regional State is in the northern part of the country, where drought and subsequent famine was common in the past. Geba catchment, the Project area is characterized by intermittent rivers which are dry 8 to 9 months with arid and semi arid climatic condition. The main socio-economic activity in the area is rain fed agriculture which directly linked to the erratic rainfall. To alleviate this rainfall dependency which is full of uncertainty the regional government has devised a mechanism by which it can supplement the rain fed agriculture with irrigation by constructing micro-dams. This task was mainly assigned for a governmental organization named Co-SEART (Commission for sustainable agriculture and environmental rehabilitation of Tigray). The activity of the organization has been stopped after constructing 60 micro-dams because it has been believed that they are less effective in supporting the rain fed agriculture through small scale irrigation schemes for institutional, technical and socio economic reasons.
Therefore the purpose of this investigation was to evaluate the role of these seemingly failed micro-dams in enhancing the surrounding groundwater system by artificial recharge. The specific objective of this study were to investigate first the interaction of these reservoir with the surrounding groundwater and if possible to quantify the total amount of recharge from the reservoir to the surrounding groundwater system.

METHODOLOGY
To achieve the above mentioned objective three wells labeled as Tsinkanet Well 1 (TSW1), Tsinkanet Well 2 (TSW2) and Rubafeleg well (RFW) and two micro dams labeled as Rubafeleg Dam (RFL) and Tsinkanet Dam (TSL) has been closely examined.
The water level fluctuation (both in the dams and wells) has been monitored for nearly two years with the average interval of 2 hours, in order to see change of the water level in © Mekelle University ISSN: 2073-073X response to rainfall and recharge. As the general purpose is to monitor the interaction between the reservoirs and the groundwater system the impact of irrigation was not considered in this investigation. has been used and for HCO 3 titration method is used and NO3, SO4, Cl, and PO4 UV spectrophotometer is used. The accuracy of the analysis has been tasted using ionic balance (Appelo, 1996). In all but one the balance is found to be less than 5%. This indicates that the analysis is sufficiently accurate. In order to interpret and classify the water a new classification scheme developed by Stuyfzand is adopted (Stuyfzand, 1986).
The classification starts with main type based on Chloride concentration, then each main type is subdivided to type based on total hardness, then each type is further subdivided into sub type based on the proportional share of main constituents in the sum of cations and anions in meq/l, finally each sub type is further classified to classes by using the following formula (Na + K + Mg) corrected = (Na + K + Mg) measured -1.061 Cl (meq/l)
Tsinkanet and Rubafeleg area are found in the upper catchment of Geba along the west and east boundaries respectively (figure 1). In both areas there is a reservoir on upstream side and shallow hand dug wells in the command areas. The UTM location of the © Mekelle University ISSN: 2073-073X reservoirs and the shallow hand dug wells where monitoring is conducted is shown on of the reservoir where as the monitoring well at Rubafeleg is more than 1 km North of the reservoirs. Both reservoirs are not more than 1 km 2 area, and there depth is not more than 10 meter. Tsinkanet Area is found between (561500 -555500 UTME and 1552100 -1546000 UTMN) and Rubafeleg area is found between (581700 -575700 UTME and 1543000 -1537500 UTMN).
The study area is accessible by all weather roads running from Hawzen to Senkata and from Atsbi to Dera.

GEOLOGY AND HYDROGEOLOGY OF THE STUDY AREA
The study area is mainly characterized by basement complex of Precambrian age, composed of meta-volcanics and meta-sediments (Kazmin, 1978). These are exposed in most parts of the mapped area. In the southwestern part a younger granite intrusion is mapped cross cutting the basement complex as shown in figure 3. Since the area is mainly characterized by crystalline metamorphic rocks, the groundwater availability and movement is mainly controlled by fractures and weathering zones. The main water supply for drinking and small scale © Mekelle University ISSN: 2073-073X irrigation is the groundwater tapped from boreholes to a maximum depth of 50 m and hand dug well with a maximum depth of 10 meter.
In Tsinkanet area the reservoir is mainly characterized by Enticho sandstone, with a thickness not more than a couple of meters. At the command area the sandstone thins out and the underlying metavolcanic rock is exposed. In Rubafeleg area both the reservoir and the command area are characterized by metamorphic rocks mainly metavolcanic and Metasediments.

Geomorphologic analysis from Topographic and Shuttle Radar Topographic Mission (SRTM, 2004) data
Topographic position is one of the governing factors that control the direction of groundwater flow. In the area the geographic location of the reservoir and the aquifer system is in a suitable location enabling the reservoir to feed the groundwater. In addition the high degree of fracturing and weathering also encourage the down gradient flow of the seepage water. Accordingly the subsurface water downstream to the reservoir is under favorable location for groundwater recharge figures 3 and 4.

Water level Analysis
Water level both in the reservoir and the surrounding groundwater was being monitored since late 2004 in Tsinkanet and Rubafeleg area. Monitoring has been conducted for more than two years till December 2006. As it can be seen on the following graphs ( Figure 6 & 7) the following observations are seen.
-The head in the micro-dam is always higher than that of the wells during the recorded period.  -The water level in the groundwater is generally found to rise, indicating a continuous accretion of groundwater from the reservoir even though the water in the wells is being continuously used for irrigation particularly during the dry season in Tsinkanet area. As the scope of this paper is to indicate the unseen role of the reservoirs as a groundwater recharging mechanism the focus is in the general trend not on quantifying the volume of water recharged.
-Marshy areas are developed downstream of reservoir.
The above observation indicates that the surface water in the reservoir to interact with the surrounding groundwater. Moreover the surface water is feeding the groundwater down stream enhancing the groundwater system as indicated by development of wet area down stream figure 5.

General chemical characteristics of the water samples in the area:
The general chemical composition of the water samples has been described with the help of  Figure   8. The samples are believed to be sufficient considering the size of the target area and the scope of the investigation. Two samples are from Rubafeleg, from the dam and borehole and two samples are from Tsinkanet, again from dam and hand-dug well. The other two samples are from river and spring in Rubafeleg area. As it has been depicted in table 2, the general characteristics of the water sample is more or less similar indicating similar origin and trend.

Stuyfzand Classification of the water samples:
To further verify similarities of the water samples from the dam and the groundwater Stuyfzand classification method has been used (Stuyfzand, 1986). Accordingly they are divided into the following.

Main type:
it is a function of Clcontent.
The chloride concentration of the samples ranges from 7.95 mg/l to 17.9 mg/l. This indicates all the samples to have a Cl concentration of less than 150 mg/l, accordingly they are grouped as fresh (F) ( Table 3).

Type (Hardness Code):
This is derived from the total hardness calculated from the following formula.

TH = 2.5 (Ca) + 4.1 (Mg)
Where TH is total hardness in mg/l and (Ca) and (Mg) are also in mg/l Accordingly the hardness of the samples is tabulated table 4 and found that they range from soft to hard water. 2. Sub Type: This is quite important to recognize the processes that have determined the water quality.
The milliequivalent concentration has been converted to proportion as Cations group and anions group. Accordingly all the samples have been found to be CaHCO3 type (Table   5).

Class (Cation Exchange Code)
The code indicates whether the cation exchange has taken place or not and also the nature of the exchange. The sum of Na, K and Mg (meq/l) is corrected for the seawater contribution, determined from the Clcontent.
(Na + K + Mg) corrected = (Na + K + Mg) measured -1.061 Cl (meq/l) (see Table 6) The Stuyfzand classification has been used to investigate see water intrusion but as the Author suggested it can also be used to determine natural waters which has undergone different processes (Stuyfzand, 1986). In general all the water samples fall in more or less similar group in which among cations Calcium is dominant and among anions HCO3 is dominant. Rubafeleg Well has relatively higher Electrical conductivity indicating that it is truly groundwater as compared to the others. Higher NO3 is observed in this well that © Mekelle University ISSN: 2073-073X could be explained with its relative position with respect to the toilet near by. From the above classification one can see that in both Rubafeleg and Tsinkanet area the water from different sources to fall in the same group with more or less similar chemical composition indicating a relationship between the dam water and the groundwater.
The chemical composition of the water could be explained by dissociation of silicate minerals mainly plagioclase feldspar and/or K feldspars as the groundwater flows through the surrounding area or as base flow water into the micro-dam. But this should be further verified by making detailed investigation on the geochemistry of the surrounding rocks.

CONCLUSION
The study tries to justify the interaction between the surface water in Tsinaknet and Rubafeleg reservoir, from physiographic, water level and hydro-chemical respect it has been found that the surface water to interact with the surrounding groundwater. In addition this reservoirs constructed for the purpose of surface water harvesting are found to enhance the groundwater system downstream encouraging the use of small hand-dug wells managed by a single family for their subsistence farming.
The study further recommend to make an investigation to quantify the quantitative recharge calculation to be made to further understand the role of this reservoir as groundwater enhancing structure using modeling approach. More over a detailed Geochemical investigation will further consolidate the findings of the research from hydrochemical point of view.