Investigating potential surface fresh water in karst Rembang using Topographic Wetness Index (TWI)

Rembang regency, Central Java Province, where annually facing drought is laid in the northern part of Java coastal area. Geographically, Rembang has various typical of landscape where affects to the water availability. There are three main problems in Rembang for water management issues, lack of quantity, quality, and continuity. To identify the water availability in order to measure the water quantity and continuity, initial research to detect the surface water in river network was carried out by applied topographic wetness index (TWI) from remote sensing, ground checked observation and focus group discussion (FGD) to find out the existing situation. The result showed, TWI was successful to detect the surface water availability in Rembang regency catchment area from its range number. Those numbers indicate the potential of surface water in focus area although the water quantity is unstable.


Introduction
Water resources management is become the most critical problem since climate change issue has been widespread over.In 2050, projected, 0.8•C to 1.4•C warm rates is increasing in Indonesia mainland, where would be impacted to the hydro-meteorological hazard event such as flood and drought frequency [1] [2].Study stated that several region in Indonesia is experiencing extreme drought caused by El-Nino Southern Oscillation (ENSO) climate anomaly that affect to the agricultural and socio-economic welfare [3] [4].Drought is defined as a natural hazard period that caused lack of water supply covers large area, it categorized into four types, they are meteorological, agricultural, hydrology, and socio-economic By the fact, integrated water resources management is highly concerned in order to fulfil the water security.Rembang regency, in Central Java Province is annually facing severe drought caused by meteorology impact, and water resource mismanagement.Rembang has two main topographic landscapes, karst in the southern area and mountainous in the northern area.These landscapes variability is impacted to the fresh water availability.In 2019, there were 16 villages hit by drought, and increase in 2020, 29 villages were hit.Base on the hydrological condition, Rembang has high potential water resources, however, it has some problems, there are (1) low quality of fresh water resources, (2) lack of fresh water quantity, (3) lack of fresh water continuity cause by natural aspects (meteorology and geology-karst) [5].
Sub-surface fresh water sources are one of the karst hydrology characteristics where difficult to detect, but can be identified from the soil characteristic and vegetation on the soil surface.The application of remote sensing by optimized topographic approaches considered to be the most relevant means of identification potential fresh water sources both in sub-surface and surface in karst topographic.Topographic wetness index is a popular approach to investigate the water flow from special catchment area that characterized by down and upper slope.The slope is extracting from Digital Elevation Model (DEM) and calculate using algorithm to model the soil moisture based on the slope [6].

Methods
TWI (topographic Wetness Index) Where, α is the local upslope area draining through a certain point per unit contour length and tanβ is the local slope.The TWI range value indicated water flow accumulation in the catchment area, where high index is recognized as high water accumulation [7].

Field survey
To validate the TWI analysis map, sets of field observation and secondary data analysis was carried out.
Figure 1.Rembang regency as focus study area 3

Result and discussion
Rembang catchment Area (DAS/Daerah Aliran Sungai) Rembang regency has thirty catchment area covered 1,035.7 km 2 wide area, however, only two catchment area has high water debit to supply domestic fresh water.They are Babadan with capacity 726 lt/sc, and Karanggeneng with capacity 1,317 lt/sc (Fig. 2).Since limited of fresh water source, there were five districts which has severe drought, Kaliori, Sumber, Bulu, Rembang, and Sulang (Fig. 3). .
A field data collection conducted to observe detail the water availability in Rembang regency and visualized to the surface water distribution map.Base on the spatial data set, more than half of Rembang regency faced the severe drought particularly in dry season [8].To detail analysed the surface water availability, we simulated the TWI to the fifth districts where has severe drought (Fig. 4 and Fig. 5).We used DEM SRTM 30 spatial resolution in order to calculate the TWI where represented the flow accumulation based on soil moisture, vegetation cover, and topography, then received to the sensor as reflection value [9].High value of TWI means there were more water accumulation, whereas low value of TWI means there were less water accumulation.We focused the analysis to the district were faced worst incident in Rembang regency since it region had large topographic spatial variation and affects to the drought pattern (table 1). Figure 4 and figure 5 showed that spatially, Rembang regency has many rivers channel and expected water availability.Figure 4 -5 showed the distribution map of TWI in five different districts in Rembang regency, which impacted drought most (Rembang, Sulang, Bulu, Sumber, and Kaliori).Based on the drought map distribution (Figure 3), Rembang district, where located in the northern part of regency and nearby the coastal line, has several channels or river that flows from the south part to the northern and the outlet are in the Java sea.These channels were identified from the blue color which means that they had high value of TWI.By understand this system, we assumed that might be the surface water is contaminated by sea water intrusion.Meanwhile, Sulang, Bulu, and Sumber districts are located in the central part of regency and placed in the mountain slope.The study showed that the streams flow accumulation are quite high where could be investigated from the blue color (high TWI). Figure 5 showed the TWI map distribution in Kaliori district where located in the coastal line as well as Rembang district.Since it location nearby the coastal, might be the surface water in Kaliori is contaminated by sea water intrusion although there are much of stream flow accumulation.
Based on the table 1, Bulu and Sumber district tends to be low of water accumulation compare to Rembang, Sulang, and Kaliori.The maximum TWI range in Bulu and Sumber districts are 20.9 and 20.8, meanwhile, Rembang, Sulang, and Kaliori district has around 22. From these result we investigated that Bulu and Sumber tends to be need more drought mitigation during dry season since the TWI range value are worst.

Conclusion
The TWI demonstrated topographical indices extracted from DEMs.The result shown location which has high potential of soil moisture based on the slope.DEM selection is adequate run in this location since Rembang regency has various topographic landscape to model TWI.High range value of TWI indicated high potential accumulation of surface water; meanwhile low range value of TWI indicated low potential accumulation of surface water.Generally, Rembang regency has high potential of surface water, however, the distribution is not spread over to the whole regions.

Figure 3 .
Figure 3. Drought map based on water availability

Table 1 .
Maximum and minimum TWI range value in fifth worst districts