Hydrology Balance in Sustainable Land Development

Several cities in West Sumatra are in an alluvial system formation even though they are at an altitude above 350 m msl, for example, Kota Solok 350 m, Bukittinggi 900 m, and Padang 25 m msl. The growth of this city was initially safe from floods, but lately, it has often experienced flood disasters or disturbed the hydrological balance. This is presumably due to changes in land use. The city of Solok has lost 385 ha of rice fields so far, the city of Padang has lost 35 ha of rice fields/year, and Bukittinggi has lost nearly 200 ha of its total rice field area. When there is a land use change to a settlement, the amount of water required during rice cultivation becomes water that has to be disposed of, and this should affect the expansion of the drainage channel. If the water requirement for one growing season is at a value of 1,300 mm, with a length of time from soil processing to harvesting is 4 months, meaning that every month the rice fields need water as much as 1,251,250 m3, which is equivalent to 0.48m3/sec.


Introduction
One of the factors assessed in the land suitability class is the hazard factor in elemental poisoning and flooding [1].Floods will reduce land productivity and increase socio-economic costs for the people who occupy the area.A settlement that floods often hit will inconvenience its residents because of the additional economic cost of goods damaged by floods.
Floods have a close relationship with water catchment areas or watersheds.Geomorphologically, the areas that always experience flooding are in the physiography of alluvial plains and intermountaneous basins.Flooding begins with the inability of the watershed to hold and absorb water, resulting in excessive surface runoff.Physiography Alluvial systems generally have pale soil color at a depth of 50 cm or generally have very poor or obstructed drainage because they have shallow groundwater depths.The shallower the groundwater needs less water for saturation, and the faster runoff occurs.
In general, the amount of water flowing in a watershed is the amount of rainfall (mm) multiplied by the catchment area (ha) multiplied by the area factor divided by the unit of time [2] .The area factor is 1 -0.65.A value of 0.65 is a mixture of hilly and mountainous areas, 0.90 is a volcanic area with primary tuffaceous parent material, and a value of 1 is for peat areas or residential areas.
The land cover factor affects the water balance in a watershed because the amount of rainfall balances with the value of evapotranspiration, water reserves in the soil, and surface runoff.If there is a reduction in land cover area and an increase in the area of open land, there will be a decrease in water storage [3].Community rubber gardens 0.37%, and built-up land 0.036% with a total area of the Barito Hulu watershed is 2,703,806.67ha.
Another researcher is Techlay Achenafy et al. who made a simulation model of static and dynamic land use in the Gumbara watershed, Ethiopia, with the land use type being 80% agriculture, 3.1% forest, 3.6% grassland, and 13.2% shrub [4].
The value of evapotranspiration will be different for each land use difference.Therefore, the essential thing in increasing the surface flow rate is the change in land use.The most extreme land-use change is when there is a change from a high water demand to zero, such as converting a rice field area into a residential or office area.
The change in wetlands is very different from dryland agriculture because wetland agriculture grows in flooded areas that require more water than dry land.It usually finds wetland farming in wet climates, with rainfall > 1500 mm/year.Furthermore, when the public converts wetland agriculture to dry land, the runoff rate will increase.In areas with general alluvial physiography bordering the sea, the groundwater level will increase due to high tides.
It mainly finds wetland agriculture in rice fields in lowland areas < 650 m, and at present, many urban development areas border these areas.The large number of wetland agricultural areas that become non-agricultural is the focus of the study in this paper.

Method and Theory
Factors that are closely related to flooding are type physiography, maximum daily rainfall, tidal conditions.Physiography that flooded constantly is Alluvial physiography and peat dome because it is closely related to the shape of the area and the depth of groundwater.This Alluvial Group has a relatively flat to slightly wavy area originating from recent and sub-recent deposits (Qal) and colluvial alluvial deposits.Dessaunett divides this alluvial group from alluvial plains transitioning to marine to flood plains from branching rivers.In mountainous areas, the area that floods often hit is the intermountaneous basin [5].
In estimating the amount of runoff related to the groundwater level and calculating the drainage channel's size, calculating the maximum daily rainfall, usually calculated based on the maximum daily data of three consecutive days, is used.Usually, the first and second days of rainfall that fall will fill and saturate the soil pores in an area, and on the third day, runoff occurs.Saturation is the amount of water required to fill the soil pores concerning the depth of the solum, calculated based on the total value of the pore space, the value of Bulk Density, the water content at pF values of 2.54 and 4.2, and the height of the groundwater table.To saturate the entire pore space, the process is necessary to fill it with water.The amount of water available on a plot of land is the difference between the water content at field capacity and the water content at the permanent wilting point multiplied by the layer thickness in mm [6].If water fills every pore, runoff occurs.Tidal conditions.Tides significantly affect the movement of water in the soil and also on the surface of rivers.If the maximum rainfall coincides with the highest tide, there will be a maximum flood height.The increase in groundwater due to water movement and rising tides results in forming of a thionic horizon in Fluvisol soils.
The survey generally determines the groundwater drainage class's depth from the results of soil surveys and mapping.The drainage class usually starts from medium, slightly obstructed, obstructed, and reduced soil in alluvial areas.Medium drainage class characterized by the presence of rust at a depth of > 80cm, slightly obstructed, rusty was found at a depth of 50-80cm and stunted, the presence of rust at a depth of 0-50 cm.A grey colour characterizes the reduced soil, usually characterized by shallow groundwater, usually 30 cm and 75 cm.In the Peat Dome area, the groundwater level is zero, meaning the soil throughout the year is in a state of water saturation.
From the FAO data [7] , the average total pore space is 38-53%, the water content at pF is 4.2, in the range of 4-17%, and the value of Bulk Density is in the range of 1.5-0.8data gr/cc.Using soil physics data and the depth of the solum, it can estimate the amount of water needed for saturation.For plant growth, it is also necessary to have a space filled with air.The amount of water required for soil saturation is 240 mm -425 mm.The process will lose the amount of water through slow drainage pores and fast drainage pores.Therefore, the water in the soil is assumed to be as much as 200 mm.
Using the average value of the FAO data [7] , the analysis can calculate the saturation value for the groundwater depth at a depth of 30 cm and 75 cm.At a depth of 30 cm, the saturation value is between 69-65 mm with an average value of 67 mm, and for a groundwater depth of 75 cm, the saturation value is in the range of 233 mm and 202 mm with an average value of 217 mm.The standard set the groundwater depths of 30 cm and 75 cm based on the criteria for soil drainage class (LPT 1969).By calculating the value of rainfall for three consecutive days and compensated for soil saturation, the result is a runoff in units of m 3 /s.

Changes in land use from rice fields to settlements
The amount of daily rainfall determines the need for the size of the sewer for at least three consecutive days, and if the data is not available, the analysis used daily data; the following table shows runoff data in several cities where floods were hitting.Based on the data in table 1, it can calculate how many volumes to design the drainage.Solok requires a size of 2  2  3m 3 , and Bukittinggi requires a size of 3  2  1 m 3 .Padang and Jakarta require canal flooding, which requires large dimensions.
Rice fields are different from dry land agriculture.Rice fields have specifications, namely plants which can grow by oxidizing the root area.In calculating the water requirement, there are also differences; in inundated soil, there is water loss through water movement in a saturated state, namely percolation and evapotranspiration, while in dryland agriculture, there is only a process of water loss through evapotranspiration.
Rice fields in Indonesia develop in wet areas.According to the climate map [8] , the area which all has wet months (CH > 100 mm) or areas with a firm, dry season with 6-7 months of dry months, i.e., months with rainfall of.60mm per month, and wet months 4-5 months.Rice fields develop in Java, around Jakarta with rainfall (3,587 mm), Semarang with rainfall (4,527 mm), and Surabaya with rainfall (4,141 mm).In Sumatra, around Medan with 3,903 mm of rainfall, around Padang with 5,563 mm of rainfall, and in Sulawesi and Makassar with 2,186 mm of rainfall.
The water requirement in paddy fields starts from the tillage phase in saturation and mudflow, vegetative growth phase, generative growth phase, maturation, and harvest phase.The water requirement in each rice field area is different in line with the difference in the soil.In the Irrigation area of Gunung Nago, the municipality of Padang, with the dominant soil is dystric Fluvisol, the water requirement for one growing season is 1.140 mm/MT [9] .Based on FAO data [7] , the assumptions are that the groundwater needs of paddy fields are in the range of 1,100 mm-1,400 mm/MT (Rasyidin 2015).When there is a change in land use from rice fields to dry land, there will be a runoff of 275 mm-338 mm.
Rice planting is done once a year in the agricultural climate zone D because in this zone, the wet months are < 3 months in a row with an annual rainfall of 2,000 mm.followed by crops for three months, and the rest follow.During the cropping of secondary crops and the fallow, the water that will become surface runoff is 600 -900 mm or the equivalent of approximately 75 mm 112.5 mm per month.On the other hand, in areas located in agricultural climate zone A with rainfall above 3,000 mm per year, farmers will usually plant rice twice a year, with a total value of water demand of 2,200 -2,800 mm in the two growing seasons.Therefore, when the farmers do not plant the land with rice, there will be a runoff in this area of 800 mm -200 mm for four months, or the equivalent of 200 mm -50 mm per month.
The runoff that occurs in always wet climate areas such as Jakarta, Semarang, Surabaya, Padang, and Medan will burden the sewer.This phenomenon may be different if found in the Mohr climate map area type 2.4, an area with a dry climate that is strict with a dry season of 6-7 months.When the analysis assesses the rainfall to be zero, the permanent sewer required is 1 lt/sec/ha -4 lt/sec/ha.Data shows that until 2001 the islands of Java and Bali had lost 1 million ha of rice fields [10] .This result means that the Java-Bali region experiences excess water of 1,000 m 3 -4,000 m 3 /month, spread over various soil and watershed areas on the island.
One example of land-use change on a small scale is the irrigation of Gunung Nago in the Lubuk Begalung District of Padang City, which experienced a decrease in the rice field area of 229.48 ha [11] .It indicated that the area would cause runoff of as much as 655,500 m 3 of water every month or require a sewer with a capacity of 0.253 m 3 /second.If there is high rainfall with an intensity of 30 mm/hour, a drain channel with a capacity of 19.17 m 3 /second will be needed, plus a regular channel of 0.25 m 3 /second.This result also means that the area requires additional channels with a capacity of 19.42 m 3 /second.Since there is no additional channel and the road area around Jalan Aru, Lubuk Begalung Village, the floods constantly fill when heavy rainfall comes.The water requirement for lowland rice is the amount of water needed during the tillage phase until the crop is harvested, including all water lost due to percolation and evapotranspiration.If there is a change in land use from wetland to dry land, the water needed for growth turns into water for drainage needs.Below is shown some data on changes in paddy fields to dry land and an increase in surface runoff.
Solok lost 385 ha out of a total area of 567 ha found in ten sub-districts within the Solok municipality.Padang has lost 1441 ha of the total area of 3000ha, namely the total irrigated area of Gunung Nago, Bukittinggi has lost 200 ha of rice fields, and Jakarta has lost 6812 ha of data Katulampa dam.
Table 2 shows that based on the area of wetland loss will cause changes in daily runoff.This study also found the most significant change in the Katulampa weir, namely 7.5 m 3 /s -91.96 m 3 /s.Using daily calculations would find the maximum value between 647 m 3 /day -795.62 m 3 /day.
When there is maximum daily rain, it must accumulate this value with the amount of runoff caused by rain and water discharged from rice fields, as shown in the following table.The data shows that Solok is in the range of 12.23-17 m3/s, Padang 43-61 m 3 /s, Bukittinggi between 5.2-9.95m 3 /s, Jakarta has a value of 344.9-428.6 m 3 /s.The accumulation of runoff in a matter of hours indicates that this value is above 40,000 m 3 /ha.Solok is at a value of 44035-44382 m 3 /hour, Padang is at a value of 155,810 -157,109 m 3 /hour, Bukittinggi is at a value of 35,833-18,889 m 3 /hour, and Jakarta is at a value of 1,238,944-124,508 m 3 /hour.A large amount of runoff water that the sewer did not accommodate causes flooding.

Conclusion
From the description above, it can draw the conclusions as follows: i.
Changes in land use significantly affect changes in the amount of surface runoff; the value of plant water needs in the area determines the magnitude of the influence is determined; ii.
The water requirement for planting in dry land is the water value needed to form dry plant material (ETc), while the water requirement for plants in wetlands, apart from the Etc value, there is also water loss through percolation; iii.
Daily data on rainfall records is crucial in calculating the amount of runoff water that occurs in the rainy atmosphere iv.
The amount of runoff water and the amount of accumulated plant water needs can be used as guidelines for making sewers so that it can obtain sustainable development in regional development; v.
Changes in rice fields into small quantities can be compensated only by making or enlarging drainage channels, while significant changes of >1000ha require constructing canals to prevent flooding and achieve sustainable land development.

Table 1 .
Total runoff at maximum daily rainfall in several cities.

Table 2 .
Amount of discharged water due to land-use change.

Table 3 .
Amount of runoff due to rainfall and excess water from paddy fields.