Mapping of rain water harvesting potential at Keduang Sub-watershed, Central Java, Indonesia

One of the main issues in agriculture in this climate change era is the water crisis. It caused a drought that affected the delay of rice cultivation. A study found that Wonogiri Regency has very critical with a value of 163.1%. Hence, it is necessary to have an alternative solution to overcome it. In this case, utilizing rain by harvesting rainwater (RWH) can be used as an alternative to meet water needs during the dry season and for various other uses. One of the RWH applications is a small-farm reservoir (SFR). To identify the potential locations for RWH is using location suitability modelling based on GIS. The determination of the location depends on biophysical criteria such as rainfall, slope, soil texture, stream order, and land use. The socio-economic attributes consist of the distance to the river, road, settlement, and agriculture. To obtain the suitability location, after all of the maps are generated, next is reclassify the value to 0 and 1. In the end, the maps calculate with a raster calculator to define the suitability location that has value 1. Location and dimension details can find out. This study aims to determine the potential of RWH for agriculture (in specific, SFR) at Keduang Sub-watershed as a consideration for stakeholders’ decision. The results showed 226 areas are potential for SFR implementation and will make it easier for stakeholders to decide by comparing the suitability location map and the long-term strategic map.


Introduction
Climate change is major problem related to water criticality, one of the main issues is the impact of El Nino. This causes drought which has an impact on delaying rice planting. Wonogiri Regency is one of the areas experiencing delays in planting, with a percentage of delay in planting 38% or 12,952 Ha of the total rice field area of 51,273 Ha [1]. Identification of water criticality in the Bengawan Solo watershed shows that Wonogiri Regency has a water availability issue [2]. Therefore, it is necessary to have an alternative solution to overcome it. In this case, utilizing rain by harvesting rainwater (RWH) can be used as an alternative to meet water needs during the dry season and for various other uses [3]. However, the development of the use of RWH is still slow. One of the problems is a lack of information. For that, it is necessary to have a system that can facilitate access to information [4].
The combination of GIS and remote sensing can determine suitable locations for rainwater harvesting. Remote sensing shows the distribution area of springs with precision via satellite. Meanwhile, the GIS gives some potential location depends on sufficient water flow to create rainwater harvesting storage [5]. To identify the potential locations for RWH is using location suitability modelling based on GIS, which combines several biophysical criteria such as slope, flow depth, land  [6]. There are many methods used in current studies to assess appropriate locations for rainwater harvesting applications. Refers to the FAO, there are six parameters to identify the location for RWH, namely: climate, hydrology, topography, agronomy, soil, and socioeconomy [7].
From 48 previous studies, the most widely used biophysical factors in identifying suitable locations for RWH in arid and semi-arid areas were slopes (83%), land cover (75%), soil type (75%), and rainfall (56%). Meanwhile, the socio-economic factors found the most influencing factors were distance to housing (25%), distance to the river (15%), distance to the road (15%), and costs (8%) [8]. One of the RWH applications is a small-farm reservoir (SFR). That is a hydrological building to hold and accommodate the flow of water sourced from springs, rainfall, rivers, and other water sources in the form of reservoirs, long-storage, and trench dams. SFR can be supplementary irrigation water during the season dry season for the food crop commodities, horticulture, plantation, and livestock [9].
Until now, no study has been found related to the potential of rainwater harvesting (RWH) potential at Keduang Sub-watershed, Central Java, Indonesia. Therefore, this study aims to determine the potential of RWH for agriculture (in specific, SFR) at Keduang Sub-watershed as a consideration for stakeholders' decision. The determination of the location depends on biophysical criteria such as rainfall, slope, soil texture, stream order, and land use. The socio-economic attributes consist of the distance to the river, road, settlement, and agriculture.

Overview of the study area
The Keduang sub-watershed is the largest in the Wonogiri watershed, with a river flow from an elevation of +1,740 m towards +139 m. River length about 45 km with an average river slope 35/1000. Geographically, the Keduang sub-watershed, Central Java Indonesia located between 7 o 42'29"-7 o 55'39" South; 111 o 11'01"-111 o 24'54" East. The Keduang sub-watershed has a slope of between 3 and 73%, with a slope of an average of 34,7% [10]. The average temperatures in 2016 between 28.35 to 28.75°C. Maximum and minimum temperatures reached 39°C and 19.28°C, respectively. The wind velocity average is about 0.96 knots, and the rain intensity average is about 16.37 mm 3 year -1 [11]. Agricultural areas in Wonogiri covered for rice fields and non-rice field farming by utilization dry fields/garden and temporarily fallow land. Rice field in Wonogiri covered about 32,677 Ha, smaller than the dry land on 88,178 Ha [11].

Datasets
The significant step in defining suitability locations in GIS is building the spatial database. This study follows various datasets, such as Digital Elevation Model (DEM), soil data, land use data, and satellitebased [12] as presented in Table 1. The software used to process, analyse and interpret the datasets is ArcGIS 10.4 and ArcSWAT, with geo-reference using WGS 1984 and zone 49S for all criteria layers.

Digital Elevation Model (DEM).
This study using DEM from DEMNAS with a spatial resolution of 8 m. Figure 1 shows the elevation of the Wonogiri Regency ranges between 0 m and 2004 m. This DEM generates a slope map and a stream order map besides obtaining the basin border and catchment area. The stream order of The Keduang sub-watershed is presented in Figure 2. The preferable stream order is small rivers (tributaries), which means 1 st and 2 nd order [9]. The slope of The Keduang subwatershed is presented in Figure 3. The recommended slope height of not more than 5% [6,12,13]. The slope height with a percentage of ≤5%, including the flat slope (plain) [14]. If it's adjusted to the standard of slope classification in the ministry of agricultural regulation of Indonesia No. 47 the year 2006, this value is equivalent to <3% with a height of <2m [15].

Rainfall data.
The rainfall map of the study site is presented in Figure 4. The Figure 4 was generated from yearly precipitation data obtained from the climatological station record for the period January-December 2016. The data then converted to shapefile and interpolated using the Isohyet method. The precipitation data from the surroundings stations is presented in Table 2.

Land use.
The land use map of The Keduang sub-watershed is presented in Figure 5 based on Rupa Bumi Indonesia (RBI). The reclassification of land use adjusts the default data from RBI [17].     Figure  6, generated from the FAO soil map. Soils with high water holding capacity are more suitable for rainwater harvesting [6].

Constraint map.
The constraint map generates through the Weighted Overlay Process (WOP) of the socio-economic data layer [7]. Figure 7-10 presents the potential distance RWH to road, river, settlement, and agriculture as the layers of constraint map using Euclidean distance. The Euclidean Distance function is used to determine the distance to roads. It is the straight-line distance between two points on a plane [12].    to 0 and 1), and determines the suitability location which is value 1 using a raster calculator. Modified from previous studies [12,13,18], the classification of the parameters presented in Table 3. The suitable location map of Keduang sub-watershed for RWH is presented in Figure 13.

Raster calculator and SWAT
The raster calculator is one of the ArcMap tools which has the function to building and executing a single Map Algebra using Python syntax in a calculator-like interface (Figure 11.). Where, Map Algebra is a simple and powerful algebra that can process all Spatial Analyst tools, operators, and functions to perform geographic analysis, which generates raster data [19]. Figure 11. Raster calculator.
The Soil and Water Assessment Tool (SWAT) was selected to simulate the hydrological processes in arid watersheds. In this study, ArcSWAT was used to calculate runoff [20,8], and to create a watershed model in determining the factors that support the calculation of SFR area and volume. These factors include runoff and evapotranspiration. This is a model that uses a combination of satellite data such as wind speed, humidity, the intensity of solar radiation, and temperature and field data such as rainfall data. Climate data is taken from Global Weather Data, while rainfall was obtained from the nearby climatology stations. Figure 12 shows the procedure of selecting potential location for RWH in Keduang sub-watershed.  Figure 12. Process of selecting potential location

SFR volume estimation
The estimation of the volume of SFR in the suitability location that resulted by raster calculator, it can be following formula (1.1). ……….……..…….. .
Where Vsfr [d] is the volume of SFR at day. Qr [d] is the daily runoff water. Asfr.R [d] is direct rainfall, where Asfr is the area of SFR. Qi [d] is the outputs consist of irrigation water. Asfr.Ev [d] is the evaporation loss [21].

Potential location for RWH
The RWH potential location map generated from various criteria, which were overlaid using the raster calculator. This map shows locations that meet the suitable criteria, thus potential for agricultural purposes-RWH, in this case, are SFR. Based on the result of the raster calculator and SWAT analysis, 226 sites potential locations have been identified for SFR implementation, both river or rainwater SFR as shown in Figure 12. In Figure  12 describes the potential for different RWH sites, depending on the suitability of the criteria in each location. In addition, it shows that for areas where the elevation is getting higher and steeper, the number of suitable sites for RWH will be less. The slope is an essential criterion in selection of location RWH buildings. High-slope areas are not suitable for large structures [12]. The areas that have moderate-high rainfall intensity also have more suitable locations [13].

SFR capacity
The volume capacity of each SFR at 226 sites ranged from 10,073.92 to 494,969.74 m 3 as presented in Table 4.