Heterogenitas Spasial Kapasitas Air Tersedia Menggunakan Digital Soil Mapping di Sub-DAS Ngrowo, DAS Brantas

Rohmatul Fadhilah, Dr. Agr. Makruf Nurudin, S.P., M.P.; Dr. Yiyi Sulaeman, S.P., M.Sc.

2026 | Skripsi | ILMU TANAH

Kapasitas air tersedia dalam tanah (Available Water Capacity) merupakan parameter penting dalam menentukan ketersediaan air dan produktivitas pertanian di Sub-DAS Ngrowo, DAS Brantas Jawa Timur yang menghadapi tekanan penurunan cadangan air tanah akibat perubahan iklim dan intensifikasi pemanfaatan lahan. Pengukuran AWC secara konvensional membutuhkan biaya mahal dan waktu yang lama, sehingga pedotransfer function (PTF) yang dikembangkan dapat menjadi alternatif meskipun akurasi pada tanah tropis masih perlu dikaji. Penelitian ini bertujuan untuk memetakan sebaran spasial AWC di Sub-DAS Ngrowo menggunakan Digital Soil Mapping (DSM), menentukan model pedotransfer paling akurat dari Model Rai, Searle dan Rawls, serta menganalisis pengaruh sifat tanah dan faktor lingkungan terhadap AWC. Penelitian dilakukan di Sub-DAS Ngrowo dengan pengamatan lapangan untuk validasi peta di 30 titik sampel tanah di Kabupaten Tulungagung dan Trenggalek pada kedalaman 0–30 cm dan 30–60 cm. Metode yang digunakan yaitu pendekatan DSM berbasis GIS, validasi lapangan, uji korelasi dan regresi. Hasil menunjukkan rata-rata AWC sebesar 2,52% (0–30 cm) dan 6,23% (30–60 cm) dengan variabilitas tinggi (CV 32,76?n 68,56%). Model Rawls terbukti paling akurat mengestimasi AWC dengan RMSE 0,87% (0–30 cm) dan 4,39% (30–60 cm) serta bias yang mendekati nol. Faktor yang mempengaruhi AWC berbeda tiap kedalaman yakni faktor pF0 pada 0–30 cm dan tekstur tanah (fraksi pasir dan lempung) pada 30–60 cm. Model Rawls direkomendasikan untuk menduga AWC di Sub-DAS Ngrowo dengan strategi pengelolaan lahan yang mempertimbangkan perbedaan karakteristik antar kedalaman tanah.

Available Water Capacity (AWC) is a critical parameter in determining water availability and agricultural productivity in the Ngrowo Sub-Watershed, Brantas Watershed, East Java, which faces increasing pressure from declining soil water reserves due to climate change and land-use intensification. Conventional AWC measurement is costly and time-consuming, making pedotransfer functions (PTFs) a potential alternative, although their accuracy in tropical soils still requires further evaluation. This study aims to map the spatial distribution of AWC in the Ngrowo Sub-watershed using Digital Soil Mapping (DSM), to determine the most accurate pedotransfer model among the Rai, Searle, and Rawls models, and to analyze the influence of soil properties and environmental factors on AWC. The research was conducted through field observations for map validation at 30 soil sampling points in Tulungagung and Trenggalek Regencies at depths of 0–30 cm and 30–60 cm, using a GIS-based DSM approach, field validation, correlation and regression analysis. The results showed a mean AWC of 2.52% (0–30 cm) and 6.23% (30–60 cm), with high variability (CV of 32.76% and 68.56%, respectively). The Rawls model proved to be the most accurate in estimating AWC, with an RMSE of 0.87% (0–30 cm) and 4.39% (30–60 cm) and bias close to zero. The factors influencing AWC differed by depth, with pF0 being the dominant factor at 0–30 cm and soil texture (sand and clay fractions) at 30–60 cm. The Rawls model is recommended for estimating AWC in the Ngrowo Sub-Watershed, supporting land management strategies that account for differences in soil characteristics across depths.

Kata Kunci : air tersedia, digital soil mapping, pedotransfer function, Sub-DAS Ngrowo