Your search found 12 records
1 Pierret, A.; Capowiez, Y.; Belzunces, L.; Moran, C. J. 2002. 3D reconstruction and quantification of macropores using X-ray computed tomography and image analysis. Geoderma, 106:247-271.
(Location: IWMI-HQ Call no: P 7565 Record No: H039066)
2 Vigiak, Olga; Ribolzi, O.; Pierret, A.; Sengtaheuanghoung, O.; Valentin, Christian. 2008. Trapping efficiencies of cultivated and natural riparian vegetation of northern Laos. Journal of Environmental Quality, 37: 889–897.
Riparian vegetation ; Sedimentation ; Rivers ; Runoff ; Control methods ; Sloping land ; Cultivation ; Rice ; Vegetables ; Teak ; Bananas ; Bamboos / Laos / Houay Pano Catchment
(Location: IWMI HQ Call no: IWMI 631.7.5 G708 VIG Record No: H040503)
In northern Laos, intensification of cultivation on sloping land leads to accelerated erosion processes. Management of riparian land may counteract the negative impacts of higher sediment delivery rates on water quality. This study assessed water and sediment concentration trapping efficiencies of riparian vegetation in northern Laos and the effect of cultivation of riparian land on water quality. Runoff flowing in and out of selected riparian sites was monitored by means of open troughs. In 2005, two native grass, two bamboo, and two banana sites were monitored. In 2006, adjacent to steep banana, bamboo, and native grass sites, three upland rice sites were established and monitored. Water trapping efficiency (WTE) and sediment concentration trapping efficiency (SCTE) were calculated on an event basis; means and 95% confi dence intervals (CIs) were estimated with a bootstrapping approach. Confidence intervals were large and overlapping among sites. Seepage conditions severely limited trapping efficiency. Native grass resulted in the highest WTE (95% CI, -0.10 to 0.23), which was not significantly different from zero. Banana resulted in the highest SCTE (95% CI, 0.06–0.40). Bamboo had negative WTE and SCTE. Median outflow runoff from rice sites was nine times the inflow. Median outflow sediment concentration from rice sites was two to five times that of their adjacent sites and two to five times the inflow sediment concentration. Although lowtillage banana plantation may reduce sediment concentration of runoff, cultivation of annual crops in riparian land leads to delivery of turbid runoff into the stream, thus severely affecting stream water quality.
3 Lacombe, Guillaume; Pierret, A.. 2011. Land cover change and catchment water yields: from local to regional scales. [Abstract only] Paper presented at the International Conference on Watershed Management - From Local Watershed Management to Integrated River Basin Management at National and Transboundary Levels, convened by the Mekong River Commission, Chiang Mai, Thailand, 9-11 March 2011. 3p.
(Location: IWMI HQ Call no: e-copy only Record No: H043679)
(0.06 MB)
Catchment water yields control the availability of the water resource and the levels of flood risk. With the demographic rise that is underway in the developing world, inter-dependencies between populations and flow variability become higher. A better management of watersheds, from local to regional levels, is therefore required.
The prerequisite for a sound catchment water management is a clear understanding of the drivers of flow variability, in relation to four main ranges of controlling factors, namely i/ the climate (rainfall-runoff relationship), ii/ water infrastructures (i.e. river flow regulated by hydropower dams), iii/ water withdrawals mainly for irrigation and iv/ land cover changes. The hydrological impacts of the first three categories of controlling factors are relatively easy to assess, as demonstrated by previous modeling efforts, noticeably in the Mekong Basin. In contrast, the way land-cover changes alter catchments’ runoff responses is less obvious and still subject to controversy.
The most reliable facts, which seem to be widely and independently acknowledged, are that deforestation, via the reduction of evapotranspiration, increases annual basin water yield, while afforestation results, over the long term, in opposite trends. These relationships were established based on analyses of hundreds of paired catchments whose surface areas rarely exceed 2 km2. The impact of land cover change on seasonal flows not only depends on evapotranspiration rates but also on soil properties such as permeability and water storage capacity, and rainfall intensities. For example, in very particular conditions, deforestation may reduce infiltration which, if not offset by a reduction in evapotranspiration, may result in reduced low flows during the dry season. For extreme flood events, the impact of reforestation may become imperceptible as over such short periods, evapotranspiration does not control the runoff response. These two examples indicate that, although deforestation and reforestation generally increases and decreases, respectively, low flows and flood peaks, the hydrological impacts of land cover changes do not follow a general rule as they depend on a complex convolution of climatic, edaphic and biological factors. However, a recent study undertaken in northern Laos in a 0.7 km2 headwater catchment under shifting cultivation corroborates most of previous results observed in other parts of the world: the development of fallow vegetation reduces groundwater recharge, leading to a drop in annual stream flow due to a decrease in wet and dry season base-flow.
Studies on the hydrological impact of land cover change over large catchments (i.e. > 1000km2) are extremely rare, in comparison with the abundance of small-scale studies. This is explained by several facts: over large areas, the heterogeneity of land covers combined with the spatial variability of climate compounds the attribution of observed hydrological changes; counteracting changes in vegetation covers may occur simultaneously and result in an apparent basin-wide stability of the runoff production. However, the sudden and irreversible bomb-induced deforestation that occurred over more than 50,000 km2 in the lower Mekong Basin during the Vietnam War has been found to have significantly increased the runoff production. This unique example of flow change induced by broad-scale deforestation in Southeast Asia demonstrates that the causal link between flow and vegetation established in small catchments can still hold over an area 4 orders of magnitude larger.
The 5 key messages:
Deforestation and reforestation increase and decrease annual water yields, respectively.
Seasonal flow response (base flow and floods) to land cover changes are less predictable and depend on climate, soil and biological conditions.
Over large scales, the detection of hydrological change is difficult, due to the heterogeneity of land cover and of their change.
Land-cover changes may have deeper hydrological impact than those expected from climate change, irrigation and hydropower development.
The inclusion of land-cover changes in the Mekong hydrological models is urgently required as land-cover change is expected to continue at a high rate over the coming decades.
The prerequisite for a sound catchment water management is a clear understanding of the drivers of flow variability, in relation to four main ranges of controlling factors, namely i/ the climate (rainfall-runoff relationship), ii/ water infrastructures (i.e. river flow regulated by hydropower dams), iii/ water withdrawals mainly for irrigation and iv/ land cover changes. The hydrological impacts of the first three categories of controlling factors are relatively easy to assess, as demonstrated by previous modeling efforts, noticeably in the Mekong Basin. In contrast, the way land-cover changes alter catchments’ runoff responses is less obvious and still subject to controversy.
The most reliable facts, which seem to be widely and independently acknowledged, are that deforestation, via the reduction of evapotranspiration, increases annual basin water yield, while afforestation results, over the long term, in opposite trends. These relationships were established based on analyses of hundreds of paired catchments whose surface areas rarely exceed 2 km2. The impact of land cover change on seasonal flows not only depends on evapotranspiration rates but also on soil properties such as permeability and water storage capacity, and rainfall intensities. For example, in very particular conditions, deforestation may reduce infiltration which, if not offset by a reduction in evapotranspiration, may result in reduced low flows during the dry season. For extreme flood events, the impact of reforestation may become imperceptible as over such short periods, evapotranspiration does not control the runoff response. These two examples indicate that, although deforestation and reforestation generally increases and decreases, respectively, low flows and flood peaks, the hydrological impacts of land cover changes do not follow a general rule as they depend on a complex convolution of climatic, edaphic and biological factors. However, a recent study undertaken in northern Laos in a 0.7 km2 headwater catchment under shifting cultivation corroborates most of previous results observed in other parts of the world: the development of fallow vegetation reduces groundwater recharge, leading to a drop in annual stream flow due to a decrease in wet and dry season base-flow.
Studies on the hydrological impact of land cover change over large catchments (i.e. > 1000km2) are extremely rare, in comparison with the abundance of small-scale studies. This is explained by several facts: over large areas, the heterogeneity of land covers combined with the spatial variability of climate compounds the attribution of observed hydrological changes; counteracting changes in vegetation covers may occur simultaneously and result in an apparent basin-wide stability of the runoff production. However, the sudden and irreversible bomb-induced deforestation that occurred over more than 50,000 km2 in the lower Mekong Basin during the Vietnam War has been found to have significantly increased the runoff production. This unique example of flow change induced by broad-scale deforestation in Southeast Asia demonstrates that the causal link between flow and vegetation established in small catchments can still hold over an area 4 orders of magnitude larger.
The 5 key messages:
Deforestation and reforestation increase and decrease annual water yields, respectively.
Seasonal flow response (base flow and floods) to land cover changes are less predictable and depend on climate, soil and biological conditions.
Over large scales, the detection of hydrological change is difficult, due to the heterogeneity of land cover and of their change.
Land-cover changes may have deeper hydrological impact than those expected from climate change, irrigation and hydropower development.
The inclusion of land-cover changes in the Mekong hydrological models is urgently required as land-cover change is expected to continue at a high rate over the coming decades.
4 Pierret, A.; de Rouw, A.; Chaplot, V.; Valentin, C.; Noble, Andrew; Suhardiman, Diana; Drechsel, Pay. 2011. Reshaping upland farming policies to support nature and livelihoods: lessons from soil erosion in Southeast Asia with emphasis on Lao PDR. [Report of the Management of Soil Erosion Consortium (MSEC) Project]. Marseille, France: Institut de Recherche pour le Developpment (IRD); Colombo, Sri Lanka: International Water Management Institute (IWMI). 40p. [doi: https://doi.org/10.5337/2011.213]
Upland areas ; Farming systems ; Shifting cultivation ; Agricultural policy ; Erosion ; Soil conservation ; Tillage ; Land use ; Site ; Monitoring ; Catchment areas ; Surface runoff ; Riparian zones ; Water management ; Vegetation ; Rain / Southeast Asia / Laos
(Location: IWMI HQ Call no: IWMI Record No: H044693)
(3.98 MB)
5 Lacombe, Guillaume; Pierret, A.. 2013. Hydrological impact of war-induced deforestation in the Mekong Basin. Ecohydrology, 6(5):901-903. [doi: https://doi.org/10.1002/eco.1395]
Deforestation ; Hydrological factors ; River basins ; Data ; Water levels ; Flow discharge ; Rain ; Land cover / South East Asia / Mekong River Basin
(Location: IWMI HQ Call no: e-copy only Record No: H045860)
(1.13 MB)
The Vietnam War played a decisive role in the pre-1990s deforestation of the lower Mekong Basin, which in turn likely influenced regional broad-scale hydrology. This note presents and discusses new analyses that strengthen this thesis. Although concurrent overestimation of discharge and underestimation of rainfall, a couple of years after bombing climaxed in the early 1970s, could theoretically explain the sharp rise in water yield previously attributed to bomb-induced deforestation, new observations suggest that bombing has durably modified the landscape: by 2002, degraded forests still largely overlapped with areas heavily bombed 30 years earlier. This corroborates observed long-term hydrological changes and suggests that warfare-induced deforestation has more profound and durable hydrological effects than previously thought.
6 Lacombe, Guillaume; Ribolzi, O.; de Rouw, A.; Pierret, A.; Latsachak, K.; Silvera, N.; Pham Dinh, R.; Orange, D.; Janeau, J.-L.; Soulileuth, B.; Robain, H.; Taccoen, A.; Sengphaathith, P.; Mouche, E.; Sengtaheuanghoung, O.; Tran Duc, T.; Valentin, C. 2015. Afforestation by natural regeneration or by tree planting: examples of opposite hydrological impacts evidenced by long-term field monitoring in the humid tropics. Hydrology and Earth System Sciences Discussions, 12:12615-12648. [doi: https://doi.org/10.5194/hessd-12-12615-2015]
Afforestation ; Plantations ; Tectona grandis ; Hydrological factors ; Humid tropics ; Ecosystem services ; Land use ; Soil conservation ; Water conservation ; Catchment areas ; Rain ; Runoff ; Models / Lao People s Democratic Republic / Vietnam / Houay Pano Catchment / Dong Cao Catchment
(Location: IWMI HQ Call no: e-copy only Record No: H047340)
http://www.hydrol-earth-syst-sci-discuss.net/12/12615/2015/hessd-12-12615-2015.pdf
https://vlibrary.iwmi.org/pdf/H047340.pdf
https://vlibrary.iwmi.org/pdf/H047340.pdf
(3.39 MB) (3.39 MB)
The humid tropics are exposed to an unprecedented modernization of agriculture involving rapid and highly-mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability controlling habitats, water resources and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydro-meteorological variables has been operating in several headwater catchments in tropical Southeast Asia since 2001. The GR2M water balance model repeatedly calibrated over successive 1 year periods, and used in simulation mode with specific rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses and trend detection tests allowed causality between land-use changes and changes in seasonal flows to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow, led to intricate flow patterns: pluri-annual flow cycles induced by the shifting system, on top of a gradual flow increase over years caused by the spread of the plantation. In Vietnam, the abandonment of continuously cropped areas mixed with patches of tree plantations led to the natural re-growth of forest communities followed by a gradual drop in streamflow. Soil infiltrability controlled by surface crusting is the predominant process explaining why two modes of afforestation (natural regeneration or planting) led to opposite changes in flow regime. Given that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.
7 Lacombe, Guillaume; Ribolzi, O.; de Rouw, A.; Pierret, A.; Latsachak, K.; Silvera, N.; Dinh, R. P.; Orange, D.; Janeau, J.-L.; Soulileuth, B.; Robain, H.; Taccoen, A.; Sengphaathith, P.; Mouche, E.; Sengtaheuanghoung, O.; Tran Duc, T.; Valentin, C. 2016. Contradictory hydrological impacts of afforestation in the humid tropics evidenced by long-term field monitoring and simulation modelling. Hydrology and Earth System Sciences, 20:2691-2704.
Humid tropics ; Hydrological factors ; Afforestation ; Natural regeneration ; Plantations ; Monitoring ; Simulation models ; Land use ; Land cover change ; Ecosystem services ; Forest conservation ; Catchment areas ; Rainfall-runoff relationships ; Stream flow ; Cropping systems ; Water conservation ; Soil conservation ; Soil surface properties ; Tectona grandis / Lao People's Democratic Republic / Vietnam / Houay Pano Catchment / Dong Cao Catchment
(Location: IWMI HQ Call no: e-copy only Record No: H047644)
http://www.hydrol-earth-syst-sci.net/20/2691/2016/hess-20-2691-2016.pdf
https://vlibrary.iwmi.org/pdf/H047644.pdf
https://vlibrary.iwmi.org/pdf/H047644.pdf
The humid tropics are exposed to an unprecedented modernisation of agriculture involving rapid and mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability which controls habitats, water resources, and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydrometeorological variables has been operating in several headwater catchments in tropical southeast Asia since 2000. The GR2M water balance model, repeatedly calibrated over successive 1-year periods and used in simulation mode with the same year of rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses, and trend detection tests allowed causality between land-use changes and changes in seasonal streamflow to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow led to intricate streamflow patterns: pluri-annual streamflow cycles induced by the shifting system, on top of a gradual streamflow increase over years caused by the spread of the plantations. In Vietnam, the abandonment of continuously cropped areas combined with patches of mix-trees plantations led to the natural re-growth of forest communities followed by a gradual drop in streamflow. Soil infiltrability controlled by surface crusting is the predominant process explaining why two modes of afforestation (natural regeneration vs. planting) led to opposite changes in streamflow regime. Given that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.
8 Ribolzi, O.; Evrard, O.; Huon, S.; de Rouw, A.; Silvera, N.; Latsachack, K. O.; Soulileuth, B.; Lefevre, I.; Pierret, A.; Lacombe, Guillaume; Sengtaheuanghoung, O.; Valentin, C. 2017. From shifting cultivation to teak plantation: effect on overland flow and sediment yield in a montane tropical catchment. Scientific Reports, 7:1-12. [doi: https://doi.org/10.1038/s41598-017-04385-2]
Teak ; Plantations ; Sediment ; Shifting cultivation ; Overland flow ; Erosion ; Slopes ; Land use ; Land management ; Flow discharge ; Vegetation ; Catchment areas ; Rainfall-runoff relationships ; Infiltration ; Soil sampling / Southeastern Asia
(Location: IWMI HQ Call no: e-copy only Record No: H048176)
(2.73 MB)
Soil erosion supplies large quantities of sediments to rivers of Southeastern Asia. It reduces soil fertility of agro-ecosystems located on hillslopes, and it degrades, downstream, water resource quality and leads to the siltation of reservoirs. An increase in the surface area covered with commercial perennial monocultures such as teak plantations is currently observed at the expanse of traditional slash-andburn cultivation systems in steep montane environments of these regions. The impacts of land-use change on the hydrological response and sediment yields have been investigated in a representative catchment of Laos monitored for 13 years. After the gradual conversion of rice-based shifting cultivation to teak plantation-based systems, overland flow contribution to stream flow increased from 16 to 31% and sediment yield raised from 98 to 609 Mg km-2. This result is explained by the higher kinetic energy of raindrops falling from the canopy, the virtual absence of understorey vegetation cover to dissipate drop energy and the formation of an impermeable surface crust accelerating the formation and concentration of overland flow. The 25-to-50% lower 137Cs activities measured in soils collected under mature teak plantations compared to soils under other land uses illustrate the severity of soil erosion processes occurring in teak plantations.
9 Manivanh, L.; Pierret, A.; Rattanavong, S.; Kounnavongsa, O.; Buisson, Y.; Elliott, I; Maeght, J. -L.; Xayyathip, K.; Silisouk, J.; Vongsouvath, M.; Phetsouvanh, R.; Newton, P. N.; Lacombe, Guillaume; Ribolzi, O.; Rochelle-Newall, E.; Dance, D. A. B. 2017. Burkholderia pseudomallei in a lowland rice paddy: seasonal changes and influence of soil depth and physico-chemical properties. Scientific Reports, 7:1-11. [doi: https://doi.org/10.1038/s41598-017-02946-z]
Pseudomonas pseudomallei ; Rice ; Soil properties ; Soil sampling ; Soil water characteristics ; Soil depth ; Chemicophysical properties ; Bacteria ; Melioidosis / Lao People's Democratic Republic
(Location: IWMI HQ Call no: e-copy only Record No: H048174)
(1.85 MB)
Melioidosis, a severe infection with the environmental bacterium Burkholderia pseudomallei, is being recognised increasingly frequently. What determines its uneven distribution within endemic areas is poorly understood. We cultured soil from a rice field in Laos for B. pseudomallei at different depths on 4 occasions over a 13-month period. We also measured physical and chemical parameters in order to identify associated characteristics. Overall, 195 of 653 samples (29.7%) yielded B. pseudomallei. A higher prevalence of B. pseudomallei was found at soil depths greater than the 30 cm currently recommended for B. pseudomallei environmental sampling. B. pseudomallei was associated with a high soil water content and low total nitrogen, carbon and organic matter content. Our results suggested that a sampling grid of 25 five metre square quadrats (i.e. 25 × 25 m) should be sufficient to detect B. pseudomallei at a given location if samples are taken at a soil depth of at least 60 cm. However, culture of B. pseudomallei in environmental samples is difficult and liable to variation. Future studies should both rely on molecular approaches and address the micro-heterogeneity of soil when investigating physico-chemical associations with the presence of B. pseudomallei.
10 Lacombe, Guillaume; Valentin, C.; Sounyafong, P.; de Rouw, A.; Soulileuth, B.; Silvera, N.; Pierret, A.; Sengtaheuanghoung, O.; Ribolzi, O. 2018. Linking crop structure, throughfall, soil surface conditions, runoff and soil detachment: 10 land uses analyzed in northern Laos. Science of the Total Environment, 616-617:1330-1338. [doi: https://doi.org/10.1016/j.scitotenv.2017.10.185]
Crops ; Soil surface properties ; Erosion ; Runoff ; Sloping land ; Food security ; Plantations ; Vegetation ; Rain ; Infiltration ; Catchment areas ; Land useCrops ; Soil surface properties ; Erosion ; Runoff ; Sloping land ; Food security ; Plantations ; Vegetation ; Rain ; Infiltration ; Catchment areas ; Land use ; Sediment ; Concentrating ; Sediment ; Concentrating / Southeast Asia / Lao People's Democratic Republic
(Location: IWMI HQ Call no: e-copy only Record No: H048341)
In Montane Southeast Asia, deforestation and unsuitable combinations of crops and agricultural practices degrade soils at an unprecedented rate. Typically, smallholder farmers gain income from “available” land by replacing fallow or secondary forest by perennial crops. We aimed to understand how these practices increase or reduce soil erosion. Ten land uses were monitored in Northern Laos during the 2015 monsoon, using local farmers' fields. Experiments included plots of the conventional system (food crops and fallow), and land uses corresponding to new market opportunities (e.g. commercial tree plantations). Land uses were characterized by measuring plant cover and plant mean height per vegetation layer. Recorded meteorological variables included rainfall intensity, throughfall amount, throughfall kinetic energy (TKE), and raindrop size. Runoff coefficient, soil loss, and the percentage areas of soil surface types (free aggregates and gravel; crusts; macro-faunal, vegetal and pedestal features; plant litter) were derived from observations and measurements in 1-m2 micro-plots. Relationships between these variables were explored with multiple regression analyses. Our results indicate that TKE induces soil crusting and soil loss. By reducing rainfall infiltration, crusted area enhances runoff, which removes and transports soil particles detached by splash over non-crusted areas. TKE is lower under land uses reducing the velocity of raindrops and/or preventing an increase in their size. Optimal vegetation structures combine minimum height of the lowest layer (to reduce drop velocity at ground level) and maximum coverage (to intercept the largest amount of rainfall), as exemplified by broom grass (Thysanolaena latifolia). In contrast, high canopies with large leaves will increase TKE by enlarging raindrops, as exemplified by teak trees (Tectona grandis), unless a protective understorey exists under the trees. Policies that ban the burning of multi-layered vegetation structure under tree plantations should be enforced. Shade-tolerant shrubs and grasses with potential economic return could be promoted as understorey.
11 Pierret, A.; Lacombe, Guillaume. 2018. Hydrologic regulation of plant rooting depth: Breakthrough or observational conundrum? Proceedings of the National Academy of Sciences of the United States of America, 115(12):E2669-E2670. [doi: https://doi.org/10.1073/pnas.1801721115]
Hydrological factors ; Plant breeding ; Roots ; Water table ; Water supply ; Waterlogging ; Nutrients
(Location: IWMI HQ Call no: e-copy only Record No: H048616)
(499 KB)
12 Ribolzi, O.; Lacombe, Guillaume; Pierret, A.; Robain, H.; Sounyafong, P.; de Rouw, A.; Soulileuth, B.; Mouche, E.; Huon, S.; Silvera, N.; Latxachak, K. O.; Sengtaheuanghoung, O.; Valentin, C. 2018. Interacting land use and soil surface dynamics control groundwater outflow in a montane catchment of the lower Mekong basin. Agriculture, Ecosystems and Environment, 268: 90-102. [doi: https://doi.org/10.1016/j.agee.2018.09.005]
Hydrology ; Land use ; Soil surface properties ; Soil crusts ; Groundwater recharge ; Flow discharge ; Water level ; Water table ; Catchment areas ; Stream flow ; Models ; Evapotranspiration ; Rain ; Runoff ; Infiltration ; River basins / China / Myanmar / Lao People's Democratic Republic / Thailand / Cambodia / Vietnam / Lower Mekong Basin
(Location: IWMI HQ Call no: e-copy only Record No: H048984)
Groundwater contribution to streamflow sustains biodiversity and enhances ecosystem services, especially under monsoon-driven climate where stream baseflow is often the only available water resource during the dry season. We assessed how land use change influences streamflow and its groundwater contribution in a small headwater catchment subject to shifting cultivation in Montane Southeast Asia. Continuous time series of rainfall, reference evapotranspiration, groundwater level, stream discharge and electrical conductivity (EC) of surface and groundwater were monitored from 2002 to 2007. With the rainfall-runoff model GR4J, we investigated temporal changes in the hydrological behaviour of the study catchment to verify consistencies with observed land use change. An EC-based hydrograph separation method allowed estimating the groundwater contribution to 104 stormflow events. Mean soil surface crusting rates corresponding to each of the nine land uses identified in the catchment were determined using 236 standard 1-m2 micro-plots. Mean plant cover for each land use was assessed in 10 × 10-m2 plots. Bedrock topography and soil layers’ structure were assessed by electrical resistivity tomography to determine pathways of subsurface storm flows. Our results indicate that an increase in the catchment's areal percentage of fallow from 33% to 71% led to a decrease in the annual runoff coefficient from 43% to 26%. The concurrent reduction of soil crusting rate over the catchment, from 48% to 30%, increased rainwater infiltration. Consecutively, groundwater contribution to storm streamflow increased from 83% to 94%, highlighting the protective role of a dense vegetation cover against flash floods. The overall reduction of the annual basin water yield for inter-storm streamflow from 450 to 185 mm suggests that the potential gain in groundwater recharge was offset by the increased root water uptake for evapotranspiration, as confirmed by the drop in the groundwater level. This analysis illustrates how two different land uses with opposite impacts on soil permeability (i/ extensive soil surface crusting under annual crops resulting in limited runoff infiltration or ii/ fallow regrowth promoting both infiltration and evapotranspiration) both inhibit groundwater recharge. The maintenance of strips of fallow buffers between annual crop plots can slow down runoff and locally promote infiltration and groundwater recharge while limiting evapotranspiration.
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