The green open space functions as a water catchment area and a source of thermal comfort

The Kalijodo green open space has an area with a 48 % vegetation area. The ideal green open space has a vegetated area of 80 to 90%. This can affect its function as a water catchment area and a source of thermal comfort. This research examines the water absorption capability and the value of the thermal humidity index in the Kalijodo green open space. The method used in this research is descriptive analysis. Based on the research results, the water absorption capability that is owned is 54,56 %. The optimum value of water absorption capability is about 75 to 95 %. The value of the thermal humidity index obtained was 30,75, and it was in a very uncomfortable category. The comfortable category of thermal humidity index is less than 29. The Kalijodo green open space does not properly function as a water catchment area and thermal comfort source. Therefore, it is necessary to improve the Kalijodo green open space condition, especially in terms of vegetation conditions.


Introduction
The Kalijodo green open space is an area with a design plan dominated by vegetated land built-in 2017 on land formerly a prostitution area to help prevent flooding, as a means of social interaction for the community and increasing the aesthetic value of the city. The ideal green open space has some functions to help absorb rainwater or surface water on the ground, thereby increasing water availability in the ground and overcoming flooding [1,2]. The ideal green open space also has other functions to reduce air temperature and reduce the effects of urban heat, thus creating an excellent urban ecology for activities and a place for social interaction and recreation [3,4,5].
Regarding the ideal green open space, which has a minimum vegetated area of 80 to 90 % of the total area of green open space, based on observations from high-resolution image interpretation results through the GIS application, Kalijodo green open space only has a vegetated area of 48 %. This result will affect the Kalijodo green open space's function, especially as a water catchment area and a source of thermal comfort. Vegetation can maintain and increase soil permeability to absorb water properly and increase the water infiltration in the soil. Vegetation can also absorb heat radiation from the sun so that it can lower the surrounding air temperature and stabilize the thermal humidity index value.
Previous research about the function of green open spaces as water catchment areas was carried out by Andini (2016) in the Kapuas square park, Pontianak. The results of observations related to the description of the green open space area show that the composition of the area of the vegetation is only 22 % of the total area of the green open space, and this is a condition that is also not ideal. Based on research related to water infiltration, the results show that the water absorption capacity that can be  [6]. The ability to absorb water is still far from the ideal value that a city park should do, which is 75 to 95 % [7]. This indicates that the Kapuas square park is not functioning correctly as a water catchment area.
Research related to the function of green open space as a source of thermal comfort has also been conducted by Sanger et al. (2016) in the city park of Bitung, North Sulawesi. The area of vegetated land-based on observations related to the area's description is 70 % of the total area of the green open space. The area of vegetation is still less than 10% of the ideal green open space criteria. The research results related to the value of the thermal humidity index in Bitung city park shows that the value is 29.39, and this value is included in the uncomfortable category [8].
Based on the results of observations and preliminary discussions on several previous studies, this study was conducted to examine and analyze the water absorption capability, in its function as a water catchment area and the value of thermal humidity index, in its function as a source of thermal comfort in the Kalijodo green open space.

Location
This research was conducted in a green open space in Kalijodo, located in the Pejagalan Village, Penjaringan District, North Jakarta Administrative City, DKI Jakarta Province. This research was conducted from June to July 2020.

Data collection technique
To obtain the data about water absorption capability discharge, the initial step that must be done is to measure the infiltration rate. Infiltration rate measurements were carried out at 3 points, with information shown in Table 1. One of the equipment used to obtain infiltration rate data is using a Double Ring Infiltrometer. After obtaining the infiltration rate data, then calculating the absorption coefficient value. The obtained absorption coefficient data are used to calculate the discharge from the potential water infiltration, determining the water absorption capability in the Kalijodo green open space. The formula for calculating the discharge from potential water infiltration [6] can be seen below: Information: P = Average annual rainfall in the last 10 years (mm/year) A = The total area (m 2 ) R = Absorption coefficient C = Runoff coefficient The activity to obtain data related to the thermal humidity index's value was carried out by measuring temperature and humidity using the Kestrel 5000 tool for five days at 5 location points in Kalijodo green open space. Measurements were made in 3 sessions per day, namely in the morning session (07.00 to 08.00 A.M.), the noon session (12.00 A.M. to 1.00 P.M.), and the afternoon session (4.00 to 5.00 P.M.). The Information about the location of temperature and humidity measurements in the Kalijodo green open space can be seen in Table 2. After measuring the air temperature, the measurement data is calculated to get the the value of thermal humidity index. The formula used to calculate the value of the thermal humidity index [9], can be seen below: Information: THI = Thermal Humidity Index RH = Rate of air humidity T = Air temperature

Data analysis technique
All of the data that has been obtained and calculated using the formulas determined, then analyzed. The analysis method used in this research is descriptive analysis. Descriptive analysis is a statistic used to analyze data by describing or describing the collected data as it is, without intending to make generalized conclusions or generalizations [10]. Descriptive analysis carried out in this study was carried out by describing and explaining the calculations' results obtained on the factors or causal variables that existed in previous theories or studies.

Location description
Based on the observations, the Kalijodo green open space has 48 % vegetated land cover, and the remaining 52 % consists of various kinds of facilities and infrastructure. More detailed information regarding the extent of land cover types in the Kalijodo green open space can be seen in Table 3.   There are two types of land cover, namely vegetated and non-vegetated land cover types. The vegetated land cover consists of grass and trees. Meanwhile, the non-vegetated land area consists of major roads, buildings, built-up land, and pedestrian paths. The total area of vegetated land cover is 16,899.41 m 2 . The total area of land cover types that are not vegetated is 18,371.41 m 2 . This clearly shows that the area of vegetated land is smaller than the land area that is not vegetated.

Water absorption capability
The Kalijodo green open space's water absorption capability is obtained based on the results of calculations from various indicators. These indicators, namely the value of the infiltration rate, the average rainfall per year, the green open space area, and the coefficient of absorption and runoff.
Measurements to obtain infiltration rate data in the Kalijodo green open space were carried out at three points (X-1, X-2, and X-3) based on pavement and canopy cover types. The measurement location point X-1 has a grass pavement type, without any canopy. The X-2 measurement location point has a type of non-grass pavement (bare soil) without any cover (canopy). The X-3 measurement location point has a grass pavement type and is under a tree canopy. The activity of measuring the infiltration rate will be carried out on June 17 th , 2020. The time to measure the infiltration rate was carried out in the dry month or dry season phase and was only done once. The infiltration rate value is obtained by calculating the measurement data using the Horton model formula. The infiltration rate values that have been obtained are classified according to Khonke (1980), ruling for further analysis. The infiltration rate values of the three measurement location points can be seen in Table 4.   Table 4 shows that the X-1 measurement location has an infiltration rate of 90,43 mm/hour. This infiltration rate is in the medium-fast category based on the classification of Kohnke (1980). The pavement is one of the factors that can affect the infiltration rate. Vegetation (the grass and the trees) has a function to increase soil permeability [1]. The laboratory tests result from soil samples were taken at the point where the measurement of the infiltration rate X-1 showed a value of 6,2 mm/hour ( Table 5). The enlargement of soil pores causes increased permeability in the soil. Soil pores also play a role in the infiltration of surface water into the soil. According to Rawls et al. (1993), where the soil is overgrown with grass, it has a better infiltration rate than non-vegetated soil and processed soil. Soils that are not vegetated and processed soils have the potential to increase soil compaction and pavement so that it has the potential to cause scale on the soil and even erosion [12].
The infiltration rate value at the point of measurement location of the infiltration rate X-2 is smallest than the location X-1 and X-3. According to Kohnke's (1980) classification, the X-2 measurement location's infiltration rate falls into the medium-slow category, with a value of 18,79 mm/hour ( Table 4). The soil's surface layer is not covered by various types of vegetation (grass and trees), causing the condition to look arid. This arid condition causes the appearance of a scale on the soil layer. The soil layer scale is caused by a compaction process, which can reduce the soil's ability to infiltrate water [12]. The laboratory test results of soil samples taken from the X-2 infiltration rate measurement location point showed a permeability value of 6,0 mm/hour, and the smallest among other soil samples ( Table 5). The soil's low ability to infiltrate water can cause standing water above the ground and potentially create flood conditions.
The infiltration rate at the X-3 infiltration rate's measurement location has a value of 284,39 mm/hour and is the largest compared to X-1 and X-2. Based on the classification of the infiltration rate by Khonke (1980), the infiltration rate value at the X-3 measurement location is included in the highspeed category. The conditions at the X-3 infiltration rate measurement location point with the surrounding land cover, full of various types of vegetation, such as grass, flowers, and trees. Variations of this type of vegetation can increase better soil permeability because of grassroots presence combined with plant and tree roots so that the soil pores are getting bigger and bigger. This is evidenced by the soil sample test results, which states that the permeability value of the soil at the measurement location of the X-3 infiltration rate has a value of 6,8 mm/hour, and this is the largest compared to X-1 and X-2 ( Table 5). The X-3 infiltration rate measurement location is perfect for draining water into the ground so that it can avoid surface waterlogging (zero runoff). Water infiltration discharge is the volume of water owned by the Kalijodo green open space in one year. In comparison, water runoff discharge is the water volume discharged by Kalijodo green open space in one year. Water infiltration discharge and water runoff discharge in the Kalijodo green open space (RTH) were obtained based on calculations from the percentage and area data of land cover and its type, absorption coefficient (R), and runoff coefficient (C) of each type of pavement [11], as well as data on the intensity of annual average rainfall in the province of DKI Jakarta in the last 10 years (2010 to 2019). The calculation of the water infiltration discharge and water runoff discharge in the Kalijodo green open space can be seen in table 6.  Table 6 shows that the water infiltration discharge rate in the Kalijodo green open space is 37.913 m 3 /year, which is the current potential water infiltration value. The discharge of water infiltration is greater than the discharge of water runoff. The Kalijodo green open space's water absorption capacity from the current absorption potential (37.913 m 3 /year) compared with the maximum absorption potential that can be obtained (69.484 m 3 /year), the result is only 54,56 %. The water absorption capability in the Kalijodo green open space is not sufficient to meet the ideal criteria of a green open space or city park, which at least can absorb water by 75 to 95 % [7]. This is due to the lack of several vegetation areas that function to increase water absorption into the soil so that the potential for water in the Kalijodo green open space is reduced.

The value of thermal humidity index
Measurement of temperature and air humidity was carried out on 6, 7, 11, 17, and 18 July 2020. Temperature and humidity data obtained from the five measurement locations in the Kalijodo green open space are then calculated to obtain the thermal humidity index's value and find out the comfort category based on the classification in Frick and Suskiyanto (2007). The value of the thermal humidity index can be seen in table 7.  Several factors other than temperature and humidity can affect the difference in the thermal humidity index's value at the temperature and humidity measurement locations in the Kalijodo green open space. Solar radiation can influence the climate. Sunlight can affect temperature and humidity in a location. High-intensity sunlight can increase air temperature and decrease humidity, thus affecting thermal comfort. Conditions also influence changes in temperature and humidity in a place. The condition in question is the existing land cover and canopy. Land cover and cover (canopy) are objects that can reflect, absorb, and even transmit radiation from sun rays [13]. An object that can radiate absorbed energy has an emissivity value. Each object also has an albedo value, which is the ratio of the radiation received to that reflected.
According to Brown and Gillespie (1995), vegetation and pavement types (cement, asphalt, buildings, stones) significantly influence the inner climate. Pavement can absorb radiation or heat from sunlight, which is higher than the type of vegetated land cover, causing air temperatures to increase and making the climate uncomfortable [13]. This is evidenced by the research results at the TP-1 temperature and humidity measurement location, which has the lowest thermal humidity index value of all measurement location points. In contrast, the TP-2 temperature and humidity measurement location have the highest thermal humidity index. The TP-1 measurement location has land cover types, namely grass, and cover (canopy), namely large trees. In contrast, the TP-2 measurement location conditions have the type of land cover, namely cement/concrete. And there is no cover (canopy).
The value of the thermal humidity index at the TP-3 temperature and humidity measurement location, shown in Table 7, is lower than that of TP-5. The conditions at the measurement location at TP-3 have cement pavement, and there is a tree cover (canopy). In contrast, the conditions at the measurement location at TP-5 has grass pavement and no cover ( canopy). This also proves that solar radiation is blocked by a cover (canopy) so that the air temperature around the TP-3 measurement location does not increase too much. Pavement conditions around the temperature and humidity measurement point of TP-3, which has grass pavement, also inhibit the rise in air temperature.
The TP-4 temperature and humidity measurement location have a thermal humidity index value that is lower than the TP-2. The condition of the TP-4 has pavement made of asphalt, and there is no cover (canopy), while the condition of the TP-2 has pavement made of cement/concrete and does not has a cover (canopy). According to Brown and Gillespie (1995), cement pavements have a higher albedo value than asphalt, thereby increasing the surrounding air temperature. The absence of a cover (canopy) at the measuring point can also increase the air temperature due to unobstructed solar radiation. Table 7 shows the average value of the overall thermal humidity index in the Kalijodo green open space, which is 30,75. This thermal humidity index value is included in the very uncomfortable