An evaluation of headwater erosion and sediment dynamics was carried out to assess the health
of the Middle Mountain agricultural system in Nepal. Controversial statements predicting this system's
imminent demise and identifying Middle Mountain farming practices as major contributors to downstream
sedimentation and flooding have long been promoted and have suggested the following research
hypothesis: soil and sediment dynamics and the indigenous management techniques within headwater
Middle Mountains basins do not indicate a deterioration in the health of the agricultural system. Three
questions were addressed in this research. What are the main controls on normal-regime erosion? How
effective is the system of indigenous management at modifying sediment dynamics? What do headwater
sediment budgets (erosion, storage, and yield) reveal about the health of the agricultural system? Answers
to these questions are suggested and development initiatives proposed.
Intensive monitoring was carried out during 1992-1994 within nested basins ranging in size from
72 to 11 141 ha. Variation of storm-period variables in time and space was assessed using five recording
rain gauges and a network of up to fifty 24-hour gauges. Surface erosion was measured from five erosion
plots on steep bari (rainfed cultivated land). Suspended sediment behaviour was examined through event
sampling at seven hydrometric stations. Basin sediment yield was determined for three of these nested
basins. Sediment storage was assessed using accumulation pins in khet fields (irrigated cultivated land),
khet canals, and bari ditches and through erosion and channel surveys.
An annual average of 77 storms were identified over the three-year period with 3.5% of these
delivering more than 30 mm total rainfall and a peak 10-minute rainfall intensity of more than 50 mm/h.
About 1/3 of all storms regardless of magnitude occurred during the pre-monsoon season. Pre-monsoon
and monsoon storms delivered equivalent high-intensity short-term rainfall disputing the hypothesis that
it is a higher rainfall intensity in the pre-monsoon season which causes an elevated sediment regime during that season. Total storm rainfall was significantly higher during the monsoon season whereas the
period without rain before a storm begins was longer for pre-monsoon storms.
The source of suspended sediment was found to vary with season and spatial scale. During the
pre-monsoon season, surface erosion from bari was severe when high-intensity rain fell on bare ground.
Indigenous farming practices were found to be effective at limiting surface erosion except during the pre-monsoon
season when targeted intervention may be useful. During the pre-monsoon season, nutrient loss
from headwater basins due to sediment export was at its highest. Severely degraded land remained bare
throughout the rainy season, producing sediment at an elevated rate and in relation to total rainfall.
The onset of the monsoon season reduced this bari source markedly due to the complete
development of a vegetative cover under conventional management. The pre-monsoon-season surface-erosion
mechanism of sediment production was replaced with scale-dependent mechanisms resulting from
the higher total rainfall of monsoon-season storms. Within the steep terraced hillslopes, the capacity of
runoff ditches was more often exceeded resulting in episodic-regime rilling, gullying, and in some
instances, terrace failure. When sufficiently heavy and widespread, monsoon storm rainfall led also to
stream discharge high enough to damage riparian areas and the system of irrigation dams.
The farmers alter the sediment regimes profoundly and their management activities reduce soil
loss collectively over all spatial scales. Sediment budgets reveal that a significant component of the
sediment produced in the study basin (5.3 km2) was recaptured (35% to 50%) because of these indigenous
farming practices. Objective calibration of indigenous knowledge showed it to be well founded but
inconsistent. Farmers practise techniques which are well adapted to this environment reflecting their stated
receptiveness to innovation and outside support.
The detailed measurements show that the important controls on erosion are variable temporally
and spatially over scales too small to be considered by conventional monitoring programs in these
environments. Spatial differences in rainfall delivery, hysteresis effects, variability in land-surface
response, and management activities conspire to yield sediment dynamics which are difficult or impossible to quantify with typical limited monitoring. Site-specific opportunities for investigation should be
exploited and a high degree of uncertainty be anticipated.
Management recommendations focus on two topics. An improved vegetative cover during the pre-monsoon
season is required to reduce soil erosion during that period. Greater retention of these nutrient-rich
soils would directly benefit the upland farmer. Rehabilitation of degraded lands and the halting or
reversing of further degradation would benefit all farmers by providing a greater land base for biomass
production especially in light of an increasing population. Both strategies would benefit hydropower
developments by limiting reservoir sedimentation. Above all, proposed changes should enhance - not
undermine - indigenous management.
Current soil dynamics may be sustainable but it is unlikely that they can remain so in the future
under the increased landuse intensification that may be necessary with projected population increases
unless support is provided strategically from outside sources. Working with the farmers to develop
techniques to improve their ability to recapture previously-eroded soil is a useful area of applied research.
The high degree of skill and adaptability of the farmers within this environment suggest that carefully
designed intervention which targets vulnerable aspects of the agricultural system while not undermining
the present methods have a reasonable likelihood for success.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/6738 |
| Date | 11 1900 |
| Creators | Carver, Martin |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Relation | UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/] |
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