Water management system of peatlands for palawija plants on KHG Pulang Pisau, Central of Kalimantan

Kesatuan Hidrologis Gambut (KHG) or Peat Hydrological Unit is a land which consist of incomplete soil decomposition of aquatic vegetation that is inundated with anaerobic conditions. Peatlands in Indonesia have common problem, which is adequacy of water availability (water balance) throughout the year and fire risk. This condition makes the holding of studies to ensure this. In this research, study uses secondary data of Peatlands, Pulang Pisau regency, Central of Kalimantan. This research will be modeling groundwater level using unconfined steady aquifer with finite difference method by Qgis Freewat. Then, if possible, the model will be calibrated by direct sampling at some point in the field, so the next will be modeling by comparison results of water level model with water level existing that up to 40 cm in accordance with Government Regulation no. 57 of 2016 on amendment to Government Regulation No. 71 of 2014 on the Protection and Management of Peat Ecosystems. From this research is expected to give some of benefits as follow: 1). Estimating water balance for Palawija Plants in KHG Pulang Pisau, 2). Modeling groundwater level for canal as one of the optimal water management efforts so that peatlands are not too dry and not too wet, so the risk of forest fires can be minimized.


Introduction
The peat ecosystem is a unit of peat element which influences each other in forming a stability, equilibrium and productivity [1]. The function will decrease if the hydrological function of the peatlands decreases. To protect it, efforts are made to plan, utilize, control, maintain, supervise and enforce the law, which is then known as the peatlands water management system. Objectives of this study are: 1) Analyzing water balance for Palawija Plants in KHG Pulang Pisau. 2) Analyzing the water level of groundwater peatlands in KHG Pulang Pisau, 3) Analyzing parameter for water managements system of peatlands in KHG Pulang Pisau.

Scope
1) Estimation of water balance in the study area based on Hydrological data at 1987 -2002. 2) Modeling groundwater level of peatlands for drainage/ canal

Methodology
This study focuses on Sub KHG 1 on Pulang Pisau Regency, Mulyasari district. This location is located between Kahayan River and Sebangau River with land area 1.786 km 2 and topographic variation 0 -10 MSL.  The sub-KHG Mulyasari above were determined based on the assumption of water dome service to the surrounding area is 30 % (flow between the Kahayan River and Sebangau River.  In the water balance analysis, what will be examined is a description of the availability of water resources based on the comparison between water requirements and water availability. So that to calculate the equilibrium of water, it is necessary to know the factors that influence the equilibrium of water in the land.
Whereas for groundwater modeling is intended to determine the behavior of groundwater movement in the hydrogeological system in the study location. Modeling will use Freewat Software with the Modflow module which is a plugin from Q Gis 2.18 Las Palmas.

Rain and climatology
In this study, the rainfall data uses rainfall data obtained in 1987-2002. Source data from Hujan, Mantaren Station. The results of regional rainfall analysis come form is thiessen polygon analysis method. and below, climatology data in area study [2].

Water balance estimation
KHG Kahayan-Sebangau is located in the delta with lowland landscapes. Based on topographic data, the results of the analysis show that there is no significant influence of water from the surrounding land into the KHG (KHG Kahayan-Sebangau), both in the form of surface flow and groundwater. The annual water balance of peatlands in the study location is dominated by rainfall as its main input, and evaporation as the main output. This output occurs through a rapid increase in debit, especially during the rainy season, while the proportion of ground water itself is quite small. In summary, this water balance can be seen in the following equation:

Modflow application
Modflow is a model of groundwater level based on the finite difference equation able to perform simulations for all types of aquifers, such as distressed aquifers, unconfined aquifers, semi-depressed aquifers, mixed aquifers [3]. The other advantages of Modflow is capable to receiving a variety of input data, the type of aquifer, layer thickness, conductuvuty hydraulics and transmissivity characteristics. In this study, the Modflow application using plugin FREEWAT by Quantum GIS version Las Palmas. The Fig. 9 is example of worksheet Q Gis using FREEWAT plugin. Groundwater flow modeling in this research using unconfined steady conditions

Hydraulic conductivity
Hydraulic conductivity has a very close relationship with the nature of aquifers. On peatlands, aquifers have a conductivity between 0.001 m / day -100 m / day. That's s value is very diverse depending on substrate roughness, decomposition, and peat humification. The forming material is unconsolidated and does not experience a large load. The existing gradient head is very low due to flat landscape.

Boundary conditions
Modeling groundwater flow for peatland uses a finite difference scheme with a grid size of 5 x 5 with ratio of 1:1. This study reviews the behavior of peat groundwater flow patterns and water level in two conditions, namely in the rainy and dry seasons with the input parameters as follows.

Rainy season
In the rainy season assumption, the results of modeling groundwater flow and water level on peatlands in Mulyasari Village as in Fig. 11.   Fig. 11. Contour water surface of peatland on rainy season (Source: Result modeling ground water peatland on rainy season, 2018) If seen in one of the grid cross sections (columns, x axis) or rows, y axis) the results can be seen in the Fig. 12 as follows.    If seen in one grid of cross section grid (column, abscissa x) or row, ordinate y) the results can be seen as in Fig. 16.   The water management system that needs to be good management. Which is set a better location for the canal blocking installation plan by focus some parameters. They are channel width, the distance between the canals, and the flowrate that exists on each channel. This is done to prevent the flow of water quickly so that it can hold water temporarily, especially on canals with relatively close distances. In addition, the potential risk of fire that will occur can be minimized.

Fire relations and drainage / canal systems
The major factor for forest fires that occurred in the study area is the behavior of land clearing, the construction of large drainage channels / canals. From these activities, it can have implications for the absorptive capacity and water reservoir in the peatlands decreasing, until the drought is not reversed.

Conclusion
Based on the research study that have been carried out, it can be concluded that. 1) The potential value of water balance contained in KHG Pulang Pisau for the characteristics of palawija crops are: at rainy season, Q 80 = 0.405 m 3 /s, and at dry season Q 80 = 0.084 m3/s 2) Water level of groundwater Peatland in KHG Pulang Pisau are: at rainy season = 0.2 m -0.35 m, and at dry season = 0.166 m -0.165 m 3) Results analyzing parameter for water managements system of Peatlands in KHG Pulang Pisau so that the peatland is not too dry and not too wet are set a better location for the canal blocking installation plan by focus some parameters. They are channel width, the distance between canals, and flowrate that exists on each channel.

Suggestion
1) Need to do the measurement of field data for calibration of modeling results. 2) Conductivity parameters of peatlands at modeling to be more variated. 3) Distance between canal to be more variated