DISERTASI ANALISIS INFILTRASI UNTUK PENETAPAN ZONASI RESAPAN AIR TANAH TIDAK TERTEKAN DI KOTA PADANG

Rusli, HAR (2024) DISERTASI ANALISIS INFILTRASI UNTUK PENETAPAN ZONASI RESAPAN AIR TANAH TIDAK TERTEKAN DI KOTA PADANG. Doctoral thesis, Universitas Andalas.

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Abstract

ABSTRACT The high rate of population growth and development in various sectors in the city of Padang, has caused changes in land use which have an impact on decreasing the quality of land resources and water resources, reducing agricultural land, decreasing catchment areas, high levels of erosion and sedimentation, increasing soil density followed by a decrease in soil permeability. The infiltration rate becomes smaller, the soil absorption capacity decreases, and the water catchment area decreases. So that surface flow increases and causes flooding in various areas in Padang City. Therefore, it is important to carry out research and analysis of infiltration characteristics to determine groundwater infiltration zoning in unconfined aquifers in Padang City. The objectives of this research are: (1) Analyzing changes in land use. (2) Analyzing rainfall and runoff discharge in Padang City. (3) Analyzing the actual infiltration characteristics in Padang City. (4) Analyze the factors that influence the infiltration rate value in Padang City. (5) Mapping the zoning of absorption areas in Padang City, and (6) Planning infiltration wells and biopores to absorb rainwater into the unconfined aquifer zone in Padang City. The research was conducted on the alluvial plains of Padang City. The area of the research area is 44.7% of the area of Padang City or 310,725 km2 . The research area is divided into 6 watersheds, namely: Btg. Kandih Watershed (67.39 km2 ), Btg. Air Dingin Watershed (131.18 km2 ), Btg. Kuranji Watershed (221.01 km2 ), Btg. Arau Watershed (178.06 km2 ), Air Timbalun Watershed (60.19 km2 ) and Sungai Pisang Watershed (37.14 km2 ). Primary data collection methods (field coordinates, infiltration rate, matric suction, soil density, hydraulic conductivity, lithology description, soil texture test and taking groundwater samples to test physical properties and type of groundwater, land slope, type and density of land cover) were carried out with a grid system. Each grid area represents one sample point for all parameters studied. The number of grids in this study is 112. The area of each grid in each watershed varies depending on the area of the watershed, the representation of all research parameters and the condition of the alluvial area in each watershed. The total area of the entire grid is 310.725 km2 . Meanwhile, secondary data was taken from several related agencies in Padang City. The data analysis method uses several approaches, regional rainfall analysis and GIS data using the Thiessen Polygon method and probability statistical analysis. The planned rain intensity is described in the form of an IDF (Intensity Duration Frequency Curve) curve, and the rain unit hyetograph is described using the Alternating Block Method (ABM). The method for estimating peak flow rates uses Hydrologic Engeneering Center – Hydrology Modeling System (HEC-HMS) modeling with the Synthetic Unit Hydrograph – Soil Conservation Service – Curve Number (HSS SCS CN) method. Analysis of runoff discharge in all watersheds in Padang City is depicted in Junction, Inflow, Outflow models and hydrograph graphs for each sub-watershed. To determine the strength of the relationship and the influence of all independent variables on the infiltration rate as the dependent variable, either partially or simultaneously, Analysis of Variance (Anova) was used with the help of the IBM version of the Statistical Package for the Social Sciences (SPSS) software. To assess the designed Ordinary Least Square (OLS) linear regression model, classical assumption tests were carried out including normality, heteroscedasticity, multicollinearity, autocorrelation, partial t test, F test and selection of the best model. The results of this research show: (1) Land use in Padang City has experienced quite significant changes. (2) Since the last 10 years, runoff discharge throughout Padang City has increased quite significantly, namely 1,103.24 m3 /second or an average increase of 37.72%. (3) The infiltration rate value throughout the Padang City area is in zone VI-E with a very low infiltration rate. The volume of rainwater that can be infiltrated is only 1,516.11 m3 /second, equivalent to 150,000 m3 /hour/km2 . The thickness of soil with low infiltration is at a depth of 0-50 cm. The initial puddle on the ground surface occurred at 5.34 minutes after the rain. (4) Partially, soil density has the greatest influence on changes in infiltration rate values. then followed in sequence from highest to lowest are: water content; matric suction; hydraulic conductivity; organic materials; soil porosity; soil texture and depth of groundwater table. Meanwhile, the variables for the physical properties of groundwater consist of: Conductivity; salinity; resistivity; DO; redox potential; pH and TDS, all of which did not make a significant contribution to changes in infiltration rate values in the study area. (5) Simultaneously, all independent variables provide correlation and influence on the dependent variable with a correlation coefficient (r) of 0.964; coefficient of determination (R2 ) = 0.929 and the contribution of all predictor variables simultaneously is 92.9%, significant at the 95% confidence level. Meanwhile, the remaining 7.1% is influenced by other variables. (6) Selecting the best model for multiple regression using the stepwise method, obtained 2 (two) best regression models, namely: the hydraulic conductivity variable and the combination of the hydraulic conductivity variable with soil porosity with the following equation: Y1 = 0.939 X1, or Infiltration Rate (f) = 0.939 Hydraulic Conductivity (K), and Y2 = 0.719 X1 + 0.297 X2, or Infiltration Rate (f) = 0.719 hydraulic conductivity + 0.297 porosity. (7) Mapping of recharge areas based on infiltration rates and the parameters that influence them in Padang City has been carried out and the results can be used as consideration for making policies in improving and handling floods in Padang City and revising the Padang City RTDR regarding groundwater infiltration zoning. (8) The use of infiltration wells and biopores to reduce excess runoff discharge and reduce flood discharge in Padang City is very ineffective, because it requires a very large number of infiltration wells and biopores and costs a lot of money. As a conversion of infiltration wells and biopores, it is best to build reservoirs with a minimum area of 1.28215 km2, spread across all watersheds in Padang City.

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