The scarcity of water in most Yogyakarta grounds water basin is because of decreasing the groundwater recharge as the implication of the build-up area growing. One exception is in Upper Belik Watershed in which more than 95% covered as paved area. In order to reduce the runoff, the infiltration well must be applied. The artificial well design depends on the characteristics of impermeable layer that can be seen by its depth. So the analysist of impermeable layer depth is extremely important to be researched. Distribution of the impermeable layer can be interpreted using Geosonar. Therefore, the purpose of this research is to find the distribution of impermeable layers in Upper Belik Catchment. The primary data used systematic random sampling base on grid size 125x125 m. Spatial data analysis of the impermeable layer utilized to visualize the result of material type which consists of sand, little gravel, tuff, clay sands, sandy clay, and gravel in research location. Variation of impermeable layer depth divided into 5 classes which are: I (9-10 m); II (10-20 m); III (20-30 m); IV (30-40 m); and V (>40m). The composition of the impermeable layer material dominated by clay and tuff in Upper Belik watershed and located below the groundwater level.

ABSTRAK

Belik Watershed is one of a catchment which located in urbanized area of Yogyakarta. The upper stream part of this watershed have very important role as recharge area. However, vast land conversion occurrence in Sleman Regency threatened its function and cause several inundation case within the area. In other words, stream capacity unable to accommodate the surface runoff. This research intend to analyzed rainfall return period and compare rainfall intensity analysis methods to be implement in the research area. Furthermore, statistical analysis performed to carry out suitable method in the previous research aims. The comparison of rainfall intensity analysis in the short time series distance utilizes Talbot Method and Sherman Method. Rainfall data of short times series distance obtained from Automatic Water Rainfall (ARR). This Calculations of the standard deviatiornesearch analyse 2, 5, 10, 25 and 50 years of return period. Calculations of the standard deviation and error square mean root peak-weight (PWRMSE) determine to choose which method has a better result. The design of rainfall intensity result shown in the Intensity-Duration-Frequency (IDF) graph. Based on the calculation, the most appropriate method is the Sherman Method with standard deviation and PWRMSE for each return period in chronological order of 2 years (-0.27) (Z 3.52); 5 years (-0.67) (Z 3.83); 10 years (0.3) (Z 5.39); 25 years (0.6) (Z 7.56); and 50 years (-1.0) (Z 9.72).