The study of water balance at Karai sub watershed

Water balance could be a clue of water available in a watershed. It could interpreted the amount of water supply and demand in an area.The extream condition of water excess or water deficiency would be flood and drought disaster. This research is aimed to obtain the value of water balance components in Karai Sub Watershed. The research result shown the source of water supply that 100 percent from rainfall was 2,811 mm/year and the total of water lost was about 1,988 mm/year that be in the form of water demand in this watershed. Several water demand in this watershed was the amount of water lost from interception about 540 mm/year, evapotranspiration about 1,382 mm/year, surface runoff abouta 42.82 mm/year, and domestic needs about 22.95 mm/year. In general, the excess water storage in Karai Sub Watershed amount 822.14 mm/year that about 29.25 percent of water supply.


Introduction
Water resources are the most important natural resources for all life human, animal, plants, and other living organism in earth.The increasing of water needs nowadays causing by the increasing of population growth, urbanization, industrial development, and increased agricultural activities.The requirement of water availability increases over time.It needs to predict the water availability.Water resource management that efficiently accurate analyses of hydrological components shown in water balance measurement that have been seen as water resource availability [1].Water balance based on conservation of mass where every changes in water content at a certain time must be equal to the difference of the amount of water added to soil and the amount of outflow water from soil.A modelling of water balance can be admitted as a equation system that considered to represent several aspect of hydrological cycle [2].
Karai Sub watershed that has a small area was the part of Ular watershed.Study on water balance is essential to be performed in a small catchment area which characterized with most of the surface rain water flows quickly into the output catchment.It will cause limited recharge of rain absorb into soil as groundwater.Meanwhile the needed of groundwater improves, the water demand-supply could be unbalance.It will causing some problems, such as seawater intrusion or water crisis [3].The imbalance of water availability and prolonged demand will impact on serious environmental problems.Studies on agricultural water balance are needed to know the availability of water resource wether the available water resources could meet the needs sufficient water for irrigation.The availability of water is an important matter to environment sustainability issue wether water is the main life source for living things.Both the excess and the deficiency of water will causing bad effects to environment such as flood and drought.Based on spatial planning, water availability will be influenced by watershed condition.Karai sub watershed was part of Ular watershed that known as one of critical watershed nowadays where the changed of landuse function occurred continuously that impacts to water balance, so that it's important to find out the availability of water in a watershed to water sustanaibility.This study aimed to obtain the value of water balance components in Karai Sub Watershed.

Materials and Methods
The research was conducted at Karai Sub Watershed that located at 98°45'3,438" East Longitude and 3°2'31,423" North Latitude.Karai Sub Watershed is upstream area of Ular Watershed.

Material
Materials used in this research are a set of computers with arcGIS software version .10.1 to create landuse map, to determine the rainfall station and, and river outlet area, Garmin GPS to obtain research coordinates, a water level logger type Global Water WLI6U to record water level data.
Rainfall data used are from Silimakuta rain gauge station year of 2021, Climatology data used obtained from Meteorological, Climatological, and Geophysical Agency, Discharge data were collected from Wampu Sei Ular Watershed Management Agency and population data were collected from Statistics Indonesia.

Determining water balance
When analyzing the water balance, we need to properly assess the water availability side and the water demand-related side of the balance.The water availability parameters of the water balance are grouped under 3 main categories, namely hydrological parameters (which include precipitation, evapotranspiration, inflow, etc.), water storage parameters (e.g.snowpack, changes in groundwater and surface water storage), and the additional water resources parameters (which include e.g. return flow, reused water, water imports, etc.).While the analysis of this parameters is done at a relevant areal unit/ scale of analysis (e.g.river basin scale), many of the data feeding these calculations are point data [4] Water balance parameters described by equation (1).P-Etc-Q-In-Qd±∆S=0 (1) P : Rainfall (mm) Etc : Evapotranspiration (mm) Q : Runoff discharge (mm) I : Interception (mm) Qd : domestics water consumption (mm) ∆S : water balance (mm) The regional average rainfall (P) was obtained by analyze rainfall data from Silimakuta Station for year 2021.Runoff discharge (Q) was obtained by analyze water level at Karai river and then the value of discharge was determined using discharge rating curve equation (2) [5].
The value of evapotranspiration was calculated using Blaney Criddle equation (3).The data used for calculate evapotranspiration was average temperature (T mean) and daily average percentage of daylight total.Eto = p (0.46 Tmean + 8.13) (3) 3 The value of interception was obtained from equation ( 4), where P is rainfall.

Precipitation
Rainfall observations (precipitation) were carried out by collecting rainfall data from the rain gauge station that is Silimakuta rain gauge, which is the only rain gauge at the research location.To calculate the water balance, basic data in the form of rainfall is needed as the main input for water supply.Large annual rainfall and a significant increase in annual average temperature will result in changes in the water balance.The monthly rainfall data (mm) can be seen in Table 1.  1 shows that there are no dry months where a dry month is defined as a month that has less than 100 mm of rainfall based on (Oldeman, 1975) [6] rainfall classification, so that all of months in this area was classified as wet month where the highest rainfall occurred in October 2021 at 430 mm/month and the lowest occurred in February at 130 mm/month.Total annual rainfall of 2,811 mm/year.The average rainfall of 234.25 mm/month is classified as moderate.Intensity of precipitation had a significant effect on water-energy balance and ecosystem recovery after drought [7].Changes in precipitation contributes to higher obscurity in ecosystem water-energy balance that requires further investigation of biomes and habitat [8].

Evapotranspiration
Determining evapotranspiration used to first analyze the amount of potential evapotranspiration.Potential evapotranspiration was obtained using the Blaney-Criddle equation.The Blaney-Criddle method requires the average temperature and the daily percentage of total daylight hours (p).Average temperature data was obtained from the Silimakuta station, while data for the daily percentage of total daylight hours (p) was obtained from the latitude of the research location and then scored using the FAO table.Evapotranspiration calculate by multipled the value of potential evapotranspiration and crop coefficient (kc) value.Crop coefficient value was obtained by calculate the crop coefficient of each land use in this area and weighted based on its area.The value of evapotranspiration present in Table 2.The relationship between evapotranspiration and water balance is that if rainfall exceeds evapotranspiration (P > ETp), there will be an increase in ground water that caused available water increased and it would be surplus water, otherwise when rainfall is less than evapotranspiration (P < ETp ), the water content in the soil will decrease and can even reach a deficit.

Interception
Interception is considered an important factor in the hydrological cycle because the reduction in rainwater reaching the ground surface due to the interception process is quite large.The interception values contained in the Karai Sub-watershed can be seen in Table 3.
Low monthly rainfall rate causing interception decrease while high rainfall rate increasing the interception value where the interception means the percentage of rainfall that can be hold by plant canopy.4. Runoff discharge cannot be separated from the influence of the shape of the watershed and also the rainfall that occurs in an area.Analyzing the shape of the watershed is very important because by knowing the shape of the watershed, water management in the watershed can be carried out optimally and avoid water overflows in the watershed.According to Susanto et al., (2017) [9] the shape of the watershed affects the concentration time of rainwater flowing into the outlet.The rounder the shape of the watershed, the shorter the concentration time required so that the higher the flood fluctuations that occur, conversely the more oval/elongated the shape of the watershed, the longer it takes.The required concentration time and flood fluctuations are lower.The watershed in the Karai Sub-watershed is oval in shape so that the concentration of flooding in the Karai Sub-watershed tends to be longer.6 that the Karai Sub-watershed has a water availability surplus of 811.46 mm/year.It shows that the water availability support the environment sustainability.The deficiency of water will causing bad effects to environment issue such as water stress and drought.The factor that most influences the water surplus is the water supply (input) that only from rainfall which is has a high rate and classified as wet in all months.The output of water is caused by plant evapotranspiration (Etc), domestic water demand (Qd), surface runoff (Qo), and interception.Rainfall (P) is higher than t evapotranspiration (Etc), domestic water requirements (Qd), surface flow (Qo), and interception so that the availability of water in the Karai Sub-watershed is able to meet daily water needs in the Karai Sub-watershed area [9], water balance is a balance that occurs between P and ETp rates.
The high rainfall rate increasing water storage and the low rainfall decreasing water storage and can reach a deficit.Between water storage and evapotranspiration (Etc) there is a general pattern that is inversely proportional, it can be seen that if evapotranspiration is high then water storage will be relatively low [9] For instance, Minderlein and Menzel reported direct influences of precipitation on soil moisture, evapotranspiration (ET), and energy balance [10].

Conclusion
Water supply at Karai Sub Watershed 100% come from precipitation amount 2811 mm/year where about 19.24% goes out as interception, 49.17% goes out as evapotranspiration, 1.56 % goes out as runoff, and 0.816 % used by domestics.In general the excess of water storage occurs amount 822.14 mm/ year that it's about 29.25%.This water excess impacts to environment sustainability

Reference [ 1 ]
Kim S, Kim J, Kang H, Jang W S and Lim K J 2021 Analysis of Water Balance Change and Parameterization Reflecting Soil Characteristics in a Hydrological Simulation Program -FORTRAN Model Water 14(6) p 990 [2] Zhang J and Liu C L 2002 Riverine Composition and Estuarine Geochemistry of Particulate Metals in China-Weathering Features, Anthropogenic Impact and Chemical Fluxes Estuarine, Coastal and Shelf Science 54 pp 1051-70 3.1.Karai Sub Watershed Climate Condition Karai Sub Watershed location was in the upstream of Ular watershed.The total area of Karai Sub Watershed is 56584.77hectares and the length of the main river is 60153.31meters.Climate condition of Karai Sub Watershed analyzed by precipitation condition in this area.The only rainfall station in this sub watershed is Silimakuta Rainfall Station which located in Simalungun Regency.The land use of Karai Sub-watershed consist of settlements covering area of 36.04 ha, forests about 69.62 ha, shrubs about 4610.74 ha, food crops about of 26,084.59ha, plantations about 14,286.31ha, and secondary forest about 11,970.28ha.

Table 1 .
The summary of monthly rainfall at Karai Sub Watershed.

Table 3 .
Interception value of Karai Sub Watershed.
3.5.Surface RunoffSurface runoff is the water that flows at a certain output point (outlet) in unit volume per time.Surface runoff shown how the watershed can hold water supply.Increasing of surface runoff shows the bad watershed health condition that means watershed couldn't hold the water supply as much which decreasing water availability.The value of surface runoff present in Table

Table 4 .
Surface Runoff of Karai Sub Watershed.Water Balance at Karai Sub Watershed Water Balance is the most important component in hydroclimatological.The value of Karai Sub Watershed water balance can be seen in Table6.

Table 5 .
Water Balance components value of Karai Sub Watershed.

Table 6
presented that there was a water deficit in 3 months, there are February that it was -7.23 mm, April that it was -7.20 mm, and June that it was -7.06 mm, while the months of water surplus occurred in January with a surplus of 36.21 mm, March of 26.93 mm, May of 110.18 mm, July of 28.86 mm, August of 183.06 mm, in September it was 76.37 mm, in October it was 227.35 mm, in November it was 105.24 mm, and in December it was 38.65 mm.The largest water deficit occurred in February at -7.20 mm, the largest water surplus occurred in October at 227.35 mm.Overall, it can also be seen in Table