HYDROLOGICAL ASPECT OF BATANG ALAI WEIR WATERSHED DUE TO LAND USE CHANGES INTO MINING AREAS

The policy of the Ministry of Energy and Mineral Resources that provides permits for mining production activities for PT. MCM in the Batang Alai River area has an impact on the hydrology of the area. This study will discuss the changes in land use in the Batang Alai River area, Batang Alai Timur District, Hulu Sungai Tengah to the aspects of high flow and low flow hydrology. The research method used is manual calculation with rational methods for high flow hydrology and calculations using the ArcSWAT 2012.10.1.18 for low flow hydrology. The data needed in the processing are rainfall data, DEM data, daily discharge data, climatology data, land use data, land use change data, and Batang Alai watershed data. The results of the study were that the ratio of peak discharge of land use was changed to the mining area and land use was not converted into a mining area for high flow hydrology calculations. There is an increase in plan flood discharge in the Batang Alai River if there is a change in land use into a mining area, the design flood discharge always increases in each period. Whereas for low flow hydrology is the comparison between the daily use of land use converted into a mining area and land use is not converted into a mining area, then the daily discharge results are calculated for the availability of water. The results of lowflow hydrological modeling are categorized as good and have high influence based on the Nash-Sutcliffe efficiency value grouping and the coefficient of determination, and water availability decreases if land use is converted into a mining area.


I. Introduction
The Changes in land use patterns (mining activities) can cause changes in ecosystems, especially in upstream areas, can have an impact on downstream areas in the form of changes in fluctuations in water discharge and sediment content and have an impact on decreasing regional water availability due to increased seasonal fluctuations with symptoms of flooding and drought.
Increasingly extreme, and the size of the watershed as well as the capacity of watershed storage systems, both on the surface (plants, rice fields, swamps, lakes/reservoirs, and rivers) and subsurface (soil and water).
This research was conducted to assess the impact of changes in land use into mining areas and to analyze their effects on the hydrological conditions that occur.

Formulation of the Problem
The things discussed in this study are as follows: 1. What is the impact if there is a change in land use (being a mining area) in the upper reaches of the Batang Alai Weir against the hydrological aspects of the area's high flow? 2. What is the impact if there is a change in land use (being a mining area) in the upper reaches of the Batang Alai Weir against the hydrological aspects of the area's low flow?

Research Purposes
The objectives of this study are: 1. Knowing the impact of land use change (being a mining area) in the upper reaches of the Batang Alai Weir on the hydrological aspects of the area's high flow (design flood discharge with return period). in the upper reaches of the Batang Alai Weir to the hydrological aspects of the area's low flow (dependable flow).

Scope of the Problem
The problem limits in this study are as follows: 1. River data collection points are in Batang Alai Timur District, Hulu Sungai Tengah (HST).
2. It doesn't count the capacity of the Batang Alai River.
3. Data collection is done by collecting hydrological data and spatial data.
4. It doesn't load the effects of erosion and sedimentation.

Definition of Hydrological Cycle
The hydrological cycle or cycle is the journey of water from the sea surface to the atmosphere than to the surface of the land, and back to the sea which never stops, the water will be temporarily held in rivers, lakes or reservoirs, and so that it can be used by humans or living things others (Asdak, 2004).

High Flow Hydrological Analysis
High flow hydrological analysis aims to determine the designed flood discharge in a watershed. Flood discharge plans can be interpreted as maximum discharge during maximum rainfall if there is not enough discharge data for analysis. Rational methods are the method most widely used for flow analysis from small watersheds. This method has a special application in urban drainage planning where peak flow calculations are used for rainwater discharge designs and small drainage structures.

Maximum Rainfall Analysis
Analysis of rainfall data is intended to determine the amount of rainfall design. This analysis includes several phases of calculation, including the rain count of the watershed area followed by an analysis of the frequency and curve of rain intensity. By calculating statistical parameters such as average values, standard deviations, the coefficient of variation, and skewness coefficients from existing data and followed by statistical tests, the appropriate probability of rain distribution can be determined. There are several distributions in the hydrological analysis including Normal distribution, Log-Normal, Extreme value Type I (Gumbel), and Log-Pearson III.

Design Rainfall Analysis with Rational Methods
Rational methods are widely used to estimate peak discharge caused

Low Flow Hydrological Analysis
Low flow hydrological analysis is a model that can simulate rain-flow transformations over a long period, rather than the period used for flood analysis. Specific models are needed in the study of low flow characteristics with an approach to the complexity of systems that exist in a watershed, then the models that are suitable for use are Even Models and Continuous Models.

Analysis of Water Availability
Changes in the relatively broad land use in the watershed area can disrupt the hydrological cycle. One method that is used to be able to find out the availability of groundwater in a watershed by comparing the value of Q max / Q min per period or also called the calculation of the Q max / Q min ratio.
Therefore to develop a comprehensive watershed management strategy that takes into account the parameters of a watershed, a hydrological model is needed that can present the hydrological cycle of a watershed.

SWAT (Soil Water and Assessment Tool)
One GIS-based software that can be used to analyze hydrological conditions is SWAT ( Q gw = Amount of water returning to groundwater on day i (mm).

General
The study was carried out for approximately 6 (six) months (January-June) in the Batang Alai watershed, which included Batang Alai Timur District, with an area of the watershed is 323,303 km 2 .
This study uses primary and secondary data derived from several government agencies and related institutions including: 1. Rainfall data.
6. Land use changes data.

Research Procedures
The steps taken in this research procedure are: 1. Research procedure for regional high flow hydrology:  Prepare data to be processed  Determine the watershed boundaries.
 Calculate the hydrological analysis. This analysis is done to get the plan discharged.
 Determine runoff discharge using rational methods. Perform calculation of peak discharge with watershed without any changes in land use into mining areas with watersheds with changes in land use into mining areas.
 Conclude from the results of the comparison of the discharge calculation.
2. Research procedures for regional low-flow hydrology:  Prepare data to be processed  Modeling with SWAT software (Soil and Water Assessment Tool):  Preparation of input data: land data, land use, climate, and DEM prepared in a database format.
 Delineate the watersheds.
 Establishment of HRU (Hydrology Response Unit Analysis) with input data on land use maps, topographic maps, and maps as well as data on the physical properties of soil.
 Climate database, by making climate generator data (weather data generator).
 Unification of HRU with climate data, this process is carried out after the analysis is formed.
 SWAT simulation is done by selecting the time to be simulated in Run SWAT mode.
 Calibration and validation of the SWAT model using daily discharge data.
 Analysis of water availability.
 Calculating water availability between watersheds that have not undergone land use change to become mines with watersheds that have experienced land use change into mining areas.

 Maximum Rainfall Calculation
The analytical methods used in this analysis are the Normal Distribution Method, Gumbel, Log-Normal, and Log Pearson Type III.

 Probability Distribution Test (Distribution Type Testing)
To strengthen the selection of the type of distribution to be used and deemed more suitable for a series of data, testing is carried out, which is commonly called: "Matching Test" ("Testing of the goodness of fit"). The test is carried out after describing the probability line on probability paper. The types of testing methods that are often carried out are the Chi-Square test and the Smirnov-Kolmogorov test.

Design flood discharge Calculation
It was carried out twice the calculation of peak discharge, first with a watershed with no changes in land use to become a mining area. And the second calculation with the watershed that has been converted into a mining area.

 Calculation of Concentration Time
Concentration-time (tc) is the length of rain that will cause flood discharge, and t is calculated by Kirpich formula (I Made Kamiana, 2010).

 Calculation of Rainfall Plan Intensity
Rainfall plan intensity is obtained from the method using the Mononobe Equation.
In this calculation, the maximum rainfall used is the maximum rainfall from the Normal Distribution Method with a 2-year return period.

 Maximum Design Discharge Calculation
In calculating the design discharge for the 2-year return period, the following results are obtained:

General Description of Batang Alai Watershed
Batang Alai watershed is located in Batang Alai Timur District, Hulu Sungai Tengah, and is included in the WGS 1984 UTM Zone 50S zone. The Batang Alai watershed area is 323,303 km², and the main river length is 48,816 km, where the topography is dominated by mountains in the southern and lowlands in the north. Batang Alai Weir is used as an outlet of this watershed.

Land Conditions
Based on the FAO soil map with a scale of 1: 5,000,000 in 1974, the type of soil found in the Batang Alai watershed consists of 2 types, namely:

Land Use
In this research, Bing 2016 imagery was used as a source of land use information. Image data is classified by dividing into several types of land use for watershed conditions without/with mining areas, forests, settlements, rice fields, and productive land.

Climate Data
The climate data of the research area is in the form of secondary climate data based on data records from the Global Weather Data for SWAT in 1995.

SWAT Analysis
In this research, a SWAT analysis using ArcSWAT 2012.10.1.18 was conducted. In this SWAT simulation, four processes were carried out, including those are: 1. the watershed delineation process, 2. establishment of a hydrological response unit (HRU), 3. SWAT data processing, 4. and the simulation process.

Results from SWAT simulations can be displayed using SWAT Plot and
Graph. Simulation output data used in this study is the surface flow and daily discharge data of the Batang Alai watershed in 1995. SWAT Plot and Graph applications can also be used to compare the discharge of simulation results with the discharge of measurement results in the field so that the model validity value can be obtained.

SWAT Model Calibration and Validation
The calibration and validation process is carried out by comparing the daily data of the observation discharge with the simulated daily discharge data for a CERUCUK Volume 4 No. 1 2020 (61-82) certain period. The statistical method used in conducting calibration and validation is the determination coefficient model (R 2 ) and Nash-Sutcliffe efficiency model (E NS ).

Water Availability Analysis
The method for estimating groundwater availability is by comparing the Q max / Q min value for each period or also called the Q max / Q min ratio calculation.  Although there was an increase in the Q max / Q min ratio from the watershed without mine and with mine, the Q max / Q min ratio of the Batang Alai watershed was still categorized as good because the ratio was less than 50. If the Q max / Q min ratio was less than 50, then the watershed was categorized as good ( Asdak, 1995).

Conclusion
Based on the analysis and discussion that has been done, conclusions can be drawn, including: 1. There is an increase in design flood discharge in the Batang Alai River if there is a change in land use to become a mining area. The design flood discharge is always increasing in each period. The design flood discharge 2-year return period for watershed without mines (Q 2 = 457,3165 m 3 / second), while the design flood discharge 2-year return period for the watershed with mines (Q 2 = 491,9183 m 3 / second) and so on. Increased the peak discharge due to changes in land use into mining areas, which also means increasing the risk of flooding in the area.
2. The calibrated model is categorized as fulfilling and having a high influence based on the grouping of Nash-Sutcliffe efficiency values and coefficient of determination. So the model can be used to simulate the desired scenario. In the period of research without mine watershed with the watershed with mining, the ratio has increased from 32,0112 to 35,5805, which means that there is a decrease in the availability of water in the watershed if there is a change in land use. But the Qmax / Qmin ratio of the Batang Alai Watershed is still categorized as good because the ratio is still less than 50.

Suggestions
The suggestions that writers can give to those who wish to continue this research or conduct similar research as follows: 1. In planning the regional spatial plan, it is necessary to review the conditions of land use more thoroughly within a certain period, so as not to cause hydrological damage to the watershed and the impact on the surrounding environment.
2. It is necessary to calculate the capacity of the Batang Alai River to find out whether or not it can accommodate flooding.
3. Study of changes in land use, runoff coefficient, runoff discharge in this study has not been supported by drainage system research. This can be continued for the next research.
4. Further studies are needed to determine the water quality in the Batang Alai watershed. This can be continued for the next research.