CHARACTERISTICS OF DUWET KARST SPRING BASED ON BASEFLOW INDEX ASSESSMENT AND ITS POTENTIAL IN FRESH WATER SUPPLY

This study aims to determine the potential of Duwet springs and their availability for supplying fresh water to the surrounding community. we use hydrographs to analyze aquifer characteristics. To analyze the hydrographs, we use water level and spring discharge data. Automatic Water Level Record (AWLR) records water level data for 1 dry season and 1 rainy season every 15 minutes. We use the volumetric method to measure the spring discharge 14 times. We compare the base flow and demand over a year to analyze the potential for fresh water. The results show that the Stage discharge rating curve y = 0.0002e5,453x with R2 value of 0.87. Duwet Springs is a perennial spring that has a small discharge (class VI). The largest discharge ever recorded was 0.69 L/s (March 7, 2020) and the smallest recorded was 0.12 L/s (August 21, 2020). BFI value varied between 0.05 and 1 with mean 0.801. The total base flow is 2490675.734 L (rainy season) and 1563419.873 L (dry season). These springs are sufficient for 75% of the rainy season and 84% in the dry season. This indicates that the existence of Duwet springs is very important for the surrounding community.


A. INTRODUCTION
Karst aquifers have a unique drainage system due to high dissolution which results the secondary porosity develops intensively. This phenomenon makes the karst area have intensive zoning system (Bakalowicz, 2005;Ford & Williams, 2007;Jennings, 1971;Sweeting, 1972). It causes the surface underground hydrological system to develop more intensively than the surface flow system (Adji & Haryono, 2017).
Therefore, dissolving is a very important part of the development of karst (karstification), compared to its relatively small role in other landforms (Ford & Williams, 2007;Gillieson, 2011;Palmer, 1999).
The intensive development of underground hydrology causes karst areas have high potential for storing water, indicated by the high water supply in the karst area which is used to fulfill the needs of fresh water. Carbonate rock aquifers in the world are able to supply 25% of the world's water needs, while their distribution is relatively small, around 12 -15.2% (Andreo et al., 2010;Ford & Williams, 2007;Goldscheider et (Adji & Haryono, 2004) that the existence is influenced by rainfall, topography, geological structure, permeability, and aquifer characteristics (Sudarmadji et al., 2012). Characteristics of karst aquifers are usually difficult to determine. Therefore, a hydrograph approach to rainfall response is important (Bonacci, 2009;Gunn, 1985;Manga, 1999;Padilla et al., 1994;Waltham et al., 1983;William B. White, 2002).

B. MATERIALS AND METHODS
The research area is situated in some part of the Gunungsewu Karst which is located in the east ( Figure 1).

Administratively, it is located in Pacitan
Regency, East Java, which is included in the Wonosari-Punung Formation. The Wonosari-Punung Formation consists of massive coral reef limestones in the south and layered limestones in the north (Balazs, 1971;Bemmelen, 1949;Surono et al., 1992;Waltham et al., 1983).
The type of soil in the study area is dominated by the lithosol mediteran and rensina complex which covers more than 90% of the total study area. The land use with a percentage of more than 50% and is followed by settlements and rainfed rice fields (Wijayanti et al., 2015). This condition is understandable given the topographical of the area which is hilly with moderate to steep slopes that limits the land use as a built-up area. The water level is obtained by using two to that proposed by (Chapman, 1991). (Chapman, 1991)   where y axis is the discharge and x axis is the water level (TMA) with an R² value of 0.87 (Figure 2). The value of minutes. An exponential regression between the water level and the discharge was carried out to be used as the basis for making hydrograph models of flow that occurred in the study area.

Index
The base flow is the flow rate in a river that is still flowing during the dry season which comes from groundwater deposits in an aquifer (Fetter, 1994). The recession coefficient in this study was 0.994. This value is used to calculate the base flow index (BFI) at Duwet Springs.
The maximum BFI is determined at 0.801. This is based on the characteristics of the research area that has perennial flow with porous rock properties (karst) in accordance with the criteria in determining BFI using the (Eckhardt, 2005) model.
The results of the calculation shows that the BFI in the study locations

Thanks to LPPM of Sebelas Maret
University for funding this Fundamental