Groundwater resources in the Middle Hills of Nepal perform a major role
in supplying domestic and irrigation water and in regulating river
flows. However, there has been little systematic study of groundwater
within the region, making it difficult to evaluate how water supplies
and river flows may change in response to climatic and anthropogenic
change. To begin to build an evidence base, two catchments in the Middle
Hills were investigated. The aim of the study was to characterise the
hydrogeology of the catchments, assess water supplies and water usage
and evaluate how resilient groundwater may be to change.
Two contrasting sub-catchments within the Kali Gandaki River catchment
were chosen: Ramche Village Development Committee (VDC), at an elevation
of 2000 – 3000 m, with subsistence terraced farming and highly forested
slopes, and Madanpokhara VDC which is largely below 1000 m, with
expanding commercial agriculture. Groundwater sampling was undertaken
during the post-monsoon season 2013 and pre-monsoon season 2014.
Springs, tube wells and rivers across the two catchments were
investigated using a combination of surveys, flow measurements, and
sampling for inorganic chemistry, stable isotopes, groundwater residence
time indicators (CFC and SF6) and noble gases. In addition, 12 months of
weekly hydrological monitoring and monthly water usage surveys were
undertaken at several sites.
There is a heavy reliance on springs for water supply in Ramche. The
springs are typically perennial but with significantly reduced flows
during the winter and pre-monsoon season. The springs have bicarbonate
groundwater chemistry and generally low overall mineralisation. Springs
issuing from the higher slopes are reliant on seasonal monsoon rainfall
and snow to sustain higher flows, but baseflows are sustained by
groundwater storage within the weathered aquifer and will therefore have
some inter-annual storage. Discrete springs issuing from lower slopes
are most likely to be fed from groundwater storage within the fractured
aquifer network. Groundwater residence time indicators (CFC and SF6)
suggest a mean residence time of 10-20 years for pre-monsoon
groundwater, implying inter-annual storage and therefore some built in
resilience. However the general low storage of the groundwater
environment suggests that none of the springs would be resilient to a
long term reduction in precipitation.
In the lower catchment of Madanpokhara where floodplain and outwash
deposits are present, many hand-drilled shallow tubewells have been
installed in the last 5-10 years, decreasing the reliance on springs.
The development of groundwater resources has resulted in a thriving
agricultural co-operative, inward migration and a growing population.
These shallow tubewells have increased the resilience of the water
supplies to change but are potentially vulnerable to over-exploitation
as a result of the rapid increase in abstraction. Groundwater sampled in
tubewells along the margin of the floodplain is modern (~20 yrs Mean
Residence Time (MRT)) with bicarbonate groundwater chemistry and no
significant water quality concerns. Groundwater sampled from tubewells
towards the centre of the floodplain appears to be older (~50 yrs MRT)
with elevated concentrations of iron, manganese, zinc and arsenic
detected at some sites.
With a growing recognition of the importance of groundwater storage in
the Middle Hills there is significant potential to further advance the
characterisation of groundwater systems and investigate the resilience
of groundwater supplies to change. Systematic monitoring of groundwater,
as springs flows, groundwater levels and chemistry would give a much
better understanding of emerging trends. Likewise, monitoring current
yields of springs and comparing to historic values at installation may
allow some conclusions to be drawn about the trajectory of springflow.
There are several groundwater-related initiatives underway within
organisations in Nepal; the lessons learned from this current research,
the methodologies used and the preliminary findings will be of value to
Bricker, S.H.; Yadav, S.K.; MacDonald, A.M.; Satyal, Y.; Dixit, A.; Bell, R. Groundwater resilience Nepal: preliminary findings from a case study in the Middle Hills. British Geological Survey, Nottingham, UK (2014) 67 pp. [British Geological Survey Open Report, OR/14/069]
Groundwater resilience Nepal: preliminary findings from a case study in the Middle Hills