More Rain, Less River Water: A Growing Himalayan Hydrology Puzzle

More rain and snowmelt do not always produce fuller rivers, according to a new analysis of watersheds across Nepal. The study, 'Discharge Deficits in the Central Himalaya Linked to Greening and Warming, published in journal nature in Scientific Reports, shows that water is taking longer to travel through many Himalayan catchments, while a growing share is being absorbed by vegetation, dry soils and deeper underground reservoirs before reaching downstream communities.

Researchers Taylor Smith and Bodo Bookhagen examined hydrological records extending from the 1960s to 2024. Their findings point to a broad change in the way Central Himalayan landscapes receive, store and release water, with consequences for ecosystems, farming, hydropower, flood behaviour and long-term water planning.

More Water Is Entering Catchments, but Less Is Reaching Rivers

The researchers began with daily discharge records from 85 monitoring stations operated by Nepal's Department of Hydrology and Meteorology. Stations containing fewer than ten complete years of observations were removed, leaving 71 long-term records covering watersheds across the country.

These measurements were compared with rainfall, snowmelt, total precipitation, temperature and evaporation estimates from the ERA5 climate dataset. The team also used MODIS satellite observations from 2001 to 2024 to track vegetation productivity and growing-season length, while other rainfall and climate datasets were examined to check whether the major patterns remained consistent.

A striking gap appeared between upstream water inputs and downstream river flow. Annual precipitation increased by an average equivalent of about 0.95 cubic metres per second each year, while measured discharge declined by around 0.28 cubic metres per second per year across the stations. Catchments receiving at least one-quarter of their flow from snowmelt showed a similar discharge decline, giving researchers little evidence that added melting was producing stronger river flows.

Snowmelt increased across much of the study area, though spring discharge continued to fall, particularly in snowmelt-influenced watersheds. High-elevation and snow-covered catchments also tended to reach 25% and 50% of their annual discharge later in the year, suggesting that early-season water is being temporarily stored or consumed before entering rivers.

Rain and Snowmelt Are Taking Longer to Become River Flow

Rainfall is positively connected with discharge during most seasons, though this relationship weakens in spring when dry soils absorb early rain, and snowmelt becomes more important. It also weakens near the end of the monsoon, when rainfall eases but water stored in saturated soils and groundwater continues feeding rivers.

The strongest rainfall-discharge connection appeared during autumn, when plant water use declines and wet post-monsoon soils allow smaller rain events to reach streams more directly. Snowmelt had its clearest influence during spring before the monsoon began. Heavy summer rainfall often overwhelmed the snowmelt signal, sometimes producing negative statistical relationships between melting and river discharge.

The researchers calculated 'transit time,' meaning the delay between rainfall or snowmelt entering a watershed and appearing as downstream flow. Rainfall transit times increased by a median of 0.05 days per year, while snowmelt transit times increased by 0.07 days per year. The long-term relationship between rainfall and discharge weakened at most stations, and snowmelt-discharge correlations also declined by a median of 0.01.

Snowmelt estimates carry greater uncertainty because rain frequently falls while snow is melting, making their separate contributions difficult to identify. The broad pattern remained visible across alternative climate datasets, corrected and uncorrected discharge records, and analyses that removed slow-moving baseflow.

Warming, Greener Slopes and Thirstier Soils May Explain the Missing Water

Satellite observations revealed increasing vegetation productivity and longer periods of active plant growth across every elevation range, with the strongest greening trends at lower elevations. Temperature rose throughout the region, with the sharpest warming found at higher elevations, while water deficits increased across all elevation groups and were most pronounced lower down the mountains.

Warmer conditions can reduce snow persistence, dry soils and allow vegetation to spread uphill into previously bare landscapes. More productive plants growing for longer periods require additional water, while depleted soils absorb a greater share of rainfall and melting snow. Water that once moved quickly into streams may now replenish soil moisture, support plant growth, evaporate back into the atmosphere or travel slowly through underground storage.

Many catchments increasingly behaved like water 'sinks,' marked by weaker precipitation-discharge links and longer transit times. These slower catchments tended to experience greater water deficits and less intense rainfall. Gentler, steadier rain is more likely to soak into soil and follow long underground routes, while intense storms can exceed the land's storage capacity and produce rapid surface runoff.

A second group became 'flashier,' showing stronger rainfall-discharge connections and shorter travel times. These watersheds were commonly associated with rising water availability and more intense precipitation. Removing baseflow reduced the dominance of the sink-like and flashy categories from 63% to 50% of the catchments, indicating that slow subsurface storage plays an important role in shaping the observed river response.

Changing River Behaviour Brings Risks and Possible Benefits

Roads, buildings, hydropower facilities, land-cover disturbance and landslides can alter how water moves through a watershed. The study compared these human and physical influences with discharge changes but found no statistically significant relationship at the national scale. A shortage of reliable information about when roads, dams and buildings were constructed made direct attribution difficult, while broad regional datasets may overlook strong effects within individual valleys.

The results suggest that warming, longer growing seasons and rising vegetation water use are more influential regional drivers than changes in snow or glacier melt. This differs from findings in parts of the northwestern Himalaya, where enhanced melting has been connected with greater early-summer discharge. Nepal's higher, sparsely vegetated and glacier-covered catchments did not show the same river-flow increase.

Slower water movement may soften discharge peaks as vegetation, soil and groundwater temporarily store rainfall, potentially reducing some flood hazards and buffering the shift from snowfall to rainfall. Lower average river flow still presents a serious concern for dry catchments, freshwater ecosystems, agriculture, drinking-water supplies and communities dependent on predictable streams.

Nepal's expanding hydropower sector makes these changes especially important. Power systems and local water plans based only on future precipitation or snowmelt could overestimate how much water will reach rivers. The research shows that planners must also consider vegetation growth, soil dryness, evaporation, groundwater storage and the changing time water spends travelling through a catchment.