Effects of (Assisted) Natural Regeneration on Infiltrability and Preferential Flow Pathways in the Khasi Hills (Meghalaya, NE India)

Intensified slash-and-burn cultivation and forest clearing have caused severe land degradation in the Khasi Hills (Meghalaya plateau, NE India). Despite very high annual rainfall, the region faces severe water scarcity during the dry season. Local initiatives aim to restore forests through assisted natural regeneration (ANR) in this hydrologically poorly known area. As a first step towards assessing the potential hydrological impact of forest regrowth and ANR, we measured infiltrability at sites representing Imperata-dominated grassland (n = 2, degraded baseline); < 20-year-old fallows with ANR (n = 3); semi-mature to old-growth forests (n = 4, near-natural baseline); and a bench-terraced Cryptomeria plantation. Group-median infiltrability was highest in forests (425 mm h⁻¹), followed by young fallows with ANR, and grasslands (145 and 68 mm h⁻¹, respectively), and the terraced plantation (50 mm h⁻¹). Infiltrability increased with regrowth age at an average rate of 3.0 mm h⁻¹ y⁻¹. Dye infiltration experiments were used to identify dominant percolation pathways per main land-cover type. Infiltration in the least-disturbed forest was dominated by flow along roots, stones and other preferential pathways, while matrix flow dominated in the grassland; the ANR site showed intermediate behaviour (macropore flow with high matrix interaction). Comparing median infiltrabilities with (maximum) hourly rainfall intensities suggested infiltration-excess overland flow may occur occasionally in the grasslands. Despite improved infiltrability during regrowth (with or without ANR), the extreme monsoon rainfall and often shallow and stony soils pose serious limitations to rebuilding soil water storage capacity and favour frequent saturation-excess overland flow.