One of the important soil characteristics is soil infiltration capacity. Soil infiltration is the maximum velocity of water entering soil profile. Soil infiltration data can be used to estimate the run off in irrigated water management and in soil and water conservation management planning. Soil infiltration capacity in a forest land is influenced by the age of plant that develop different composition of forest land communities. The study aimed to observe soil infiltration capacity at various age classes of teak (Tectona grandis L.) stands. The infiltration rate in each location was measured by a double ring infiltrometer. The results showed that the infiltration capacity rate increased as the teak plant age increased. The older the teak plant age the higher the organic matter content and soil porosity. This has good influences in improving soil structure and texture as well as improving soil macro fauna activities in soil surface; accordingly soil capacity rate will increase. Thinning in the fifth age class and girdling in the eight age class caused forest vegetation more open. This will increase soil compaction and will decrease soil infiltration capacity.

Water quality and quantity are important factors for water resources evaluation. Water quality and quantity should be measured directly. Unfortunately, some watersheds do not have hydrologic stations. Water quantity can be estimated through modeling. One of the simple models for monthly water potential estimation is Thronthwaite-Mather method. The method is based on water balance in which rainfall as an input, and evapotranspiration and run-off as outputs. Physical soil properties and land cover characteristics especially rootzone are as processors. The aim of this research was to compare between estimation and direct measurement of monthly water potential. The result showed that monthly run-off prediction has high correlation with direct measurement (R 2 = 0.77-0.91). In order to get an appropriate result, the method should reconsider about water surplus assumption. The assumption of 50% water surplus for next month will not be suitable for all watersheds. Some watersheds may assume more than 50% water surplus and some other watersheds may assume less than 50%, depending on watershed characteristics, especially on geological formation. In order to get a more accurate result, the direct measurement of run-off should be rechecked. ABSTRAKKualitas dan kuantitas air adalah faktor penting dalam evaluasi sumberdaya air. Kualitas dan kuantitas air harus diukur secara langsung. Namun demikian, tidak semua DAS memiliki stasiun hidrologi. Kuantitas atau jumlah air dapat dihitung dengan pemodelan. Salah satu model yang sederhana untuk memperkirakan potensi air bulanan adalah metode Thornthwaite-Mather. Metode ini didasarkan pada neraca air. Hujan sebagai masukan, evapotranspirasi dan debit sebagai luaran. Sifat fisik tanah dan karakteristik penutupan lahan sebagai pemroses. Tujuan penelitian ini adalah untuk mendapatkan informasi tentang hasil perbandingan antara penaksiran potensi air bulanan dan hasil pengukuran langsung. Hasil penelitian menunjukkan bahwa penaksiran debit bulanan mempunyai korelasi yang tinggi dengan hasil pengukuran dengan R 2 berkisar antara 0,77 sampai 0,91. Dalam rangka mendapatkan hasil yang sesuai maka metode ini perlu mempertimbangkan perubahan asumsi surplus air. Asumsi surplus air 50% untuk bulan berikutnya kurang cocok untuk semua DAS. Beberapa DAS mungkin mempunyai asumsi surplus air lebih dari 50%, sedangkan beberapa DAS lainnya mungkin kurang dari 50%, tergantung pada karakteristik DAS, khususnya formasi geologi. Untuk mendapatkan hasil pembanding yang baik, maka hasil pengukuran langsung sebaiknya dilakukan pengecekan lagi.Kata kunci: Estimasi potensi air, neraca air, pengukuran langsung neraca air