Sri Lanka; Impact of Rainfall Runoff Harvesting in Drought Prone Areas


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Sri Lanka; Impact of Rainfall Runoff Harvesting in Drought Prone Areas

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Sri Lanka; Impact of Rainfall Runoff Harvesting in Drought Prone Areas

  1. 1. IMPACT OF RAINFALL RUNOFF HARVESTING IN DROUGHT PRONE AREAS -A CASE STUDY IN KOTEWEHARA OF SRI LANKA C.Shanthi de Silva1 and Tanuja Ariyananda2 ABSTRACTThe case study area is located in the border of Intermediate zone, dry zone boundary of Sri Lanka. Theannual rainfall in the study area is about 1200 mm which is distributed over few months of April, andMay and then October to December. Generally more than 4 months of the year do not receive rainfall.People in the study area face drought during in dry season. Some of the houses have modern rain waterharvesting systems in use. Most of the systems are in use, mainly for domestic consumption as potablewater. Water shortage is the major problem and the people in the area are mainly depend on thegroundwater as there are few surface irrigation tanks which do not hold enough water for agriculturalactivities. Groundwater too is limited in the study area and the wells get dried during the drier monthsand the rural population is unable to maintain even a small vegetable garden for their ownconsumption. Therefore, this study was focused on the possibility of raising the groundwater levels bystoring rainfall runoff water in open dug wells (pathahas), specialized runoff collection tanks andexisting village tank (wawe) during the rainy season in the vicinity of the wells. Therefore the peoplein the area could have water for cultivation during the dry period. In the case study area 8 wells wereselected in random around the existing village tank and the rainfall runoff collection structures such asPathahas and Runoff collection tanks and the Indian artificial recharging structure were constructed inthe vicinity of the wells. Water levels were monitored daily in all the rainfall runoff collectionstructures and the wells. In addition to the water levels, rainfall and evaporation were also measureddaily.A Soil and moisture balance based spread sheet model (SAMBA) was used to estimate the rechargeand runoff in the study area. The study results has shown a significant response to the water level in theshallow wells in the vicinity of the recharge structures such as pathahas and runoff collection tanks andvillage tank. It shows that the recharge to the shallow wells by the rainfall runoff collected in thestructures is effective to keep the water level high during the dry season. All the wells in the study areaexcept the deeper wells (well no 5) were maintained at the average water depth below ground level of 4m which is adequate to maintain a small vegetable garden during dry season for their ownconsumption. In previous years the water level had dropped below 6 meter of the well depth during thedry months. The local community in the area too convinced that the water levels in wells weremaintained high after the construction of rainfall runoff collection structures.INTRODUCTIONThe dry and intermediate zones of Sri Lanka face severe water shortage during dry season from Marchto September. More than 70 % of the annual average rainfall in these zones is received during a shortperiod of 3 to 4 months from October to January. During this period there is significant amount ofrunoff due to the low infiltration rates through the weathered overburden above the crystalline hardrock aquifer in these dry and intermediate zones of Sri Lanka (Figure 1). If this rainfall runoff iscollected effectively on surface it will contribute during dry season from March to September. Themain occupation and source of income of the farmers in these zones is farming.__________________________________________________________________________________1 Professor in Agricultural and Plantation Engineering, Faculty of Engineering Technology, The Open University of SriLanka, Nawala, Nugegoda2 Director, Lanka Rainwater Harvesting Forum, 28/3 A, Subadrarama Lane, Nugegoda, Sri Lanka 1
  2. 2. Each farmer owns land of 2 to 3 hectares (Irrigation Department Report, 2003). Farmers in this areamainly cultivate Beet, Onions, Tomato, Brinjal, Chilli and Capsicum as mixed cropping. The land areadoes not seem to be a limiting factor for farming in this area. The major limiting factor for farming isthe availability of wateras their wells get dried in mid of July. Therefore the aim of this paper to study the impacts of rainfallrunoff collection in the vicinity of wells to maintain high water table.METHODOLOGYIn the case study area eight agro wells were selected randomly. Three dug wells (pathahas) wereconstructed near Well No 1, 2 and 3 to collect the runoff water in the vicinity. In addition three runoffcollection tanks were constructed near well no 4, 5 and 6. Water levels in the wells, pathahas andrunoff collection tanks were monitored daily. Figure 2 shows the location of the wells. Table 1 showsthe detail dimensions of the well and the runoff collection structures. Daily rainfall and temperaturealong with other met data were collected from the nearby meteorological Station. RESULTS AND DISCUSSIONRainfall RunoffRainfall is one of the climatic parameter which contributed significantly to this study (Figure 3a).Generally the months of July, August and September do not receive any rainfall in the study area. Therainfall distribution has two peaks; the months April, May and June received rainfall of 48 mm, 157mm and 150 mm respectively. Jaffna Trincomalee Anuradhapura A dry zone B intermediate zone Puttlam C wet zone study area A Kurunagala B C Colombo deep sedimentary rocks Hambantota shallow sedimentary strata 0 100 km crystaline hard rocks 2
  3. 3. Figure 1 Climatological Zones and Aquifer types in Sri Lanka Upland permanent grass W7 and vegetables Lowland paddy W6 Tank Upland permanent grass W2 Lowland and vegetables W1 paddy 0 500 m W3 Figure. 2 Location of wells, paddy fields and grassland in the study area Table 1 Details of the Wells and the Recharging structures Name of the Well Runoff Collection Structures Elevation* (m) Owner No Depth(m) No Depth(m) Meewallawe Wawe 4.4 15.0 H.H Dinadasa 01 7.3 Pathaha 1 19.06 H.G Herath Banda 02 8.6 Pathaha 2 20.71 M.Harshana 03 4.1 Pathaha 3 19.67 Susantha M.Mudiyansa 04 6.4 Runoff Collection Tank 2.2 15.78 J.H.Chandrasekera 05 11.5 Runoff Collection Tank 2.9 15.03 H.H Karunaratne 06 10.9 Runoff Collection Tank 2.5 19.68 H. Appuhamy 07 7.0 13.27 R.H.Jayathilke 08 6.6 16.07*Elevations were measured from a high point of the study areaSimilarly the months of September, October, November and December received 93 mm, 197mm, 280mm and 153 mm respectively. These two peaks of rainfall contributed would have contributed to thesignificant amount of rainfall runoff. The modified soil moisture balance method (Rushton 2003) wasused to estimate the runoff in the study area. The total runoff estimated during the study period was157 mm (Figure 3). 3
  4. 4. RechargeResults show that the recharge occurred in three main periods during May to June and from October toDecember 2004. The modified soil moisture balance estimated potential recharge was 191 mm (Figure3b). Recharge was calculated using water table fluctuation method. For the water table fluctuationmethod the ground water level rise due to rainfall was carefully analyzed and multiplied by the specificyield value of the aquifer. The specific yield value of 0.065 which was calculated by the pumping studyanalysis in the similar area near to the study area was used (De Silva and Rushton, 1996). The averagewater table rise in the tank and runoff collection structures during the recharge periods wasapproximately 2 m. Therefore the recharge estimated from the tank water rise was 130 mm during thestudy period. The estimated recharge in wells varied from 280 mm to 130 mm which was higher theaverage recharge value of the study area.Water level response in runoff collection structures and wellsTank water level was maintained at 2.5 m below ground level from March 2004 to end of April 2004and increased up to 1.75m below ground level with the rainfall in April and May 2004. From June2004 the tank water level gradually decreased and reached 4m below ground level in end of October2004. With the wet season rains the tank water level increased to 2 m below ground level andmaintained in that level till end of May 2005 (Figure 3c& 3d).All three pathahas were located near well no 1, 2 and 3 (all these three wells are in the same elevation)water levels in these pathahas were increased during the period of May to September 2004 while theTank was losing water. It may be due to the fact that the tank water is recharging the pathaha due to itsshallow depth (Figure 3). Consequently the shallow well no 3 (well depth is 4 m) water level wasmaintained in average depth of 3 m below ground level may be due to the recharging effect of thePathaha in the vicinity of this well. Recharge contribution from the three Pathaha to well no 1 and 2and less because these two wells are deeper than the well no 3. It shows that the tank contributesrecharge to pathahas and pathahas contribute recharge to wells of shallow depth. Since the data wascollected only for a year the effect on deep wells are less noticeable. But it may contribute in the longrun.The runoff collection tanks 1, 2 and 3 were located near the wells no 4, 5 and 6 respectively. The waterlevel in the runoff collection tank no 1 located near to the well no 4 was maintained near to the groundlevel after the wet season rains in September, October, November and December (Figure 3). Thewater in the runoff collection tank may have contributed recharge to the well no 4 which is of shallowin depth of 6.4 m compared to the well no 5 (well depth is 11.5 m). Because of it the water level in thewell no 4 was maintained at the average of 4 m below ground level. Contribution of the water in therunoff collection tank 2 and 3 were not prominent. CONCLUSIONSThe preliminary study on the effect of recharging the groundwater through the structures such aspathahas and runoff collection tanks has shown a significant recharge to the well in the vicinity.Recharge contribution to the wells of shallow depth were prominent than the deep wells. It shows that 4
  5. 5. (a) Runoff estimated by modified soil moisture balance method 80 0 Rainfall (mm) Runoff (mm) 60 10 40 20 20 0 30 1M 31 M 30A 30Ma 29J 29Ju 28A 27S 27O 26N 26D 25Ja 24FRunoff = 157 mm DateRainfall =1339 mm RAIN RO(b) Recharge calculated by the modified soil moisture balance method 40 Recharge (mm) 30 20 10 0 1M 31 M 30A 30Ma 29J 29Ju 28A 27S 27O 26N 26D 25Ja 24F DateRecharge = 191.4mm(c) Water levels in runoff collection structures Date 1- 20- 8- 27- 17- 5- 24- 13- 1- 20- 9- 28- 16- 4- 23- 12- 31- 19- 8- 27- Jan Jan Feb Feb Mar Apr Apr May Jun Jun Jul Jul Aug Sep Sep Oct Oct Nov Dec Dec 0 80 Water level (cm) 100 60 200 40 300 400 20 500 0 RF P2 RWC1 TANK(d) Water levels in shaloow and deep wells Date 1- 22- 12- 4- 25- 15- 6- 27- 17- 8- 29- 19- 9- 30- 21- 11- 2- 23- Jan Jan Feb Mar Mar Apr May May Jun Jul Jul Aug Sep Sep Oct Nov Dec Dec 0 200 Water level (cm) 400 600 800 1000 1200 Well 2 Well 3 Well 4 Well 6Figure 3 Runoff, Recharge and water levels in wells and runoff collection structures 5
  6. 6. the recharge from these structures contributes to the shallow wells effectively. But it may take longtime to contribute to the deeper wells. It is also evident that it is important to collect run off water instructures such as pathaha and runoff collection tanks to maintain the water levels in the wells high. Allthe wells in the study area except the deep wells (well no 5) maintained at the average water depthbelow ground level of 4 m which is desirable for year around cultivation. If not the water level dropsbelow 6 meters of the well depth as in well no 5 during August, September and October whichundesirable for agricultural activities.Estimated recharge in tank, pathahas, and runoff collection tanks by the water table fluctuation methodwas 130. The modified soil balance methods estimated recharge and runoff were 190 mm and 157 mmrespectively. The recharge estimated in runoff collection structures by the water table fluctuationmethod was 130mm. The estimated recharge in wells by the water table fluctuation method varied from280 mm to 130 mm which was higher the average recharge value of the study area. The resultsconfirmed the higher recharge to keep the well water level high as 4 m below ground level through outthe dry season was due to the runoff collection structures such as tank, pathahas and runoff collectiontanks in the vicinity of the shallow wells. It is early to decide which runoff collection structurerecharging structure is the best because the data was collected only for one year and there will bevariation over the period of years. REFERENCESDe Silva, C.S (1995). The use of agrowells for supplementary irrigation from hard-rock aquifers in SriLanka. Ph.D Thesis , Silsoe College, Cranfield University , UKDe Silva, C.S (1997). Groundwater Regulation through Design of Agrowell Dimension in the hard-Rock Aquifer- A Case Study in the North Western province of Sri Lanka, Tropical AgriculturalResearch, Vol 9, 288-301.De Silva, C.S and Rushton, K.R (1996) Interpretation of the behavior of agrowell systems in Sri Lankausing radial flow models. J. of Hydrological Sci. 41(6): 825-835Kolrkar, A.S., Murti,K.N.K, and Singh, N. (1980). Water harvesting and runoff farming in aridRajasthan. Indian J.Soil Cons. (2): 113-119.Rushton, K.R. (2003) Groundwater Hydrology, Conceptual and Computational Models. Wiley,Chichester, 416pp.Vijayalakshmi, K., Vittal, K.P.RandSingh, R.P. (1982). Water harvesting and recycling. In “Decade ofDry land Agricultural Research in India 1971-80’.AICRPDA, Hyderabad, India. p 45-58 6