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Do forests have an impact on water availability?: Assessing the effects of heterogeneous land use on streamflow in two monsoonal river basins
Linköping University, The Tema Institute, Department of Water and Environmental Studies. Linköping University, Faculty of Arts and Sciences.
2000 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to assess the effects of land use changes on streamflow in two river basins, the upper Bhavani in south India and the upper Nam Pang in northeast Thailand. In the Nam Pang basin, the forest cover has decreased from 80% to 27% in the last 30 years. Despite this, almost no changes in streamflow patterns or amountswere found. The figures depicting a drastic reduction of indigenous forest are partly misleading. In areas, where swidden agriculture has been the cause of forest encroachment, large numbers of shade trees were retained thus the density of trees in the catchment has not been as radically reduced (219 trees ha·1 to 104 trees ha-1 ) as the amount of forest cover indicates. Many abandoned plots of land, have also been rapidly replaced with secondary vegetation, which attain evapotranspiration rates close to that of mature forests in only a few years. This would indicate that substituting indigenous forest witha mosaic of open land and mixed trees does not affect the streamflow amounts as drastically as has been observed in small catchments where an area of forest is cleared simultaneously and replaced with homogeneous cropland.

People in both catchments valued trees highly for productivity functions such as firewood, food items, medicines and aesthetic reasons. Forests were also believed very closely linked with a sustained water availability in terms of rain and streamflow. Because of how highly forests are valued, there was a strong interest in both conserving the indigenous forests that still exist today as well as retaining and planting scattered trees. This would aid the maintenance of a landscape mosaic that should according to the results presented in this thesis, not drastically affect streamflow regimes from more heavily forested conditions.

Study work in the upper Bhavani catchment, India, was riddled with data uncertainties that made modelling work wrought with extra challenges. Even in areas such as this, where data is insufficient in relation to the area's hydrological and climatological complexities, people have an interest in understanding their local hydrological regime. It is therefore justifiable to model these areas, if the available data is assessed until an acceptable level of reliability is obtained. Results should then be presented and interpreted in light of these data uncertainties. Results from the modelling of different land use scenarios supported the results from the upper Nam Pang catchment, Thailand where more heterogeneous land use conditions, showed little changes in streamflow regimes compared to a hypothetical indigenous scenario. Most extreme changes in annual water yield were caused by the scenarios representing total conversion of the catchment to agriculture (+19%) and plantations (-33%) while changes in assured yield at the Bhavanisagar reservoir, a measurement indicating downstream water sustainability, were more modest.

In summary, the retention of heterogeneous land use can buffer the effects of large changes in streamflow as found in small-scale catchment studies. It is very likely that people that enter a forested area to undertake small-scale agriculture will maintain existing forests and plant scattered tree groves for the many products and services that trees are perceived to provide, thus propagating a landscape mosaic.

Place, publisher, year, edition, pages
Motala, Sweden: Motala Grafiska , 2000. , p. 79
Series
Linköping Studies in Arts and Science, ISSN 0282-9800 ; 222
National Category
Social Sciences Interdisciplinary
Identifiers
URN: urn:nbn:se:liu:diva-31783Local ID: 17607ISBN: 91-7219-882-6 (print)OAI: oai:DiVA.org:liu-31783DiVA: diva2:252606
Public defence
2000-12-08, Hörsal Planck, Fysikhuset, Universitetsområdet Valla, Linköping, 10:15 (Swedish)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2018-01-13Bibliographically approved
List of papers
1. GIS-supported modelling of areal rainfall in a mountainous river basin with monsoon climate in southern India
Open this publication in new window or tab >>GIS-supported modelling of areal rainfall in a mountainous river basin with monsoon climate in southern India
2000 (English)In: Hydrological Sciences Journal, ISSN 0262-6667, E-ISSN 2150-3435, Vol. 45, no 2, p. 185-201Article in journal (Refereed) Published
Abstract [en]

Spatial rainfall patterns and seasonal variability were assessed for a mountainous river basin with monsoon climate. Factors were identified that could explain this variability, and a GIS-supported method to determine the areal distribution of precipitation was developed. To find acceptable regression equations, a division had to be made between rainfall stations dominated by the southwest-monsoons and the northeast-monsoons, respectively. Distance to the southwestern border was the main explaining factor for precipitation at southwest-monsoon dominated stations. For northeast-monsoon dominated stations, altitude and slope were the most important factors. The basin was divided into pixels with characteristics typical for northeast- or southwest-monsoon dominated rainfall stations to allow calculation of spatial rainfall. The difference when comparing regression-based estimates with Thiessen-based estimates was small when considering the annual estimates for the whole basin. However, when analysing seasonal rainfall or sub-catchments, the differences between Thiessen-based and regression-based estimates were significant.

Abstract [fr]

La structure spatiale et la variabilité saisonnière de la pluviosité ont été déterminées pour un bassin versant montagneux soumis à un climat de mousson. Les facteurs susceptibles d'expliquer cette variabilité ont été identifiés et une méthode assistée par SIG de détermination de la distribution spatiale des précipitations a été développée. Afin d'obtenir des équations de régression acceptables nous avons dû distinguer les stations pluviométriques soumises à la mousson du SO de celles soumises à la mousson du NE. La distance à la frontière SO s'est révélée être le principal facteur explicatif des précipitations pour les stations soumises à la mousson de SO. Pour les stations soumises à la mousson de NE, l'altitude et la pente se sont révélées être les facteurs explicatifs principaux. Le bassin a été divisé en pixels auquels les caractéristiques des stations soumises à la mousson du NE ou du SO ont été affectées pour pouvoir calculer la pluie spatiale. Les différences entre les estimations fondées sur les régressions et celles fondées sur la méthode de Thiessen sont petites lorsque l'on considère le bassin complet à l'échelle annuelle, mais elles peuvent devenir importantes lorsque l'on considère les sous-bassins à l'échelle saisonnière.

National Category
Social Sciences Interdisciplinary
Identifiers
urn:nbn:se:liu:diva-31760 (URN)10.1080/02626660009492319 (DOI)17583 (Local ID)17583 (Archive number)17583 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2018-01-13Bibliographically approved
2. Hydrological impacts of forest conversion to agriculture in a large river basin in northeast Thailand
Open this publication in new window or tab >>Hydrological impacts of forest conversion to agriculture in a large river basin in northeast Thailand
2001 (English)In: Hydrological Processes, ISSN 0885-6087, E-ISSN 1099-1085, Vol. 15, no 14, p. 2729-2748Article in journal (Refereed) Published
Abstract [en]

Small-scale experiments have demonstrated that forest clearance leads to an increase in water yield, but it is unclear if this result holds for larger river basins (> 1000 km(2)). No widespread changes in rainfall totals and patterns were found in the 12100 km(2) Nam Pong catchment in northeast Thailand between 1957 and 1995, despite a reduction in the area classified as forest from 80% to 27% in the last three decades. Neither were any detectable changes found in any other water balance terms nor in the dynamics of the recession at the end of the rainy season. When a hydrological model calibrated against data from the period before the deforestation was applied for the last years of the study period (1987-1995), runoff generation was however underestimated by approximately 15%, indicating increased runoff generation after the deforestation. However, this was mainly due to the hydrological response during one single year in the first period, when the Q/P ratio was very low. When excluding this year, neither analysis based on the hydrological model could reveal any significant change of the water balance due to the deforestation. More detailed land-use analysis revealed that shade trees were left on agricultural plots as well as a number of abandoned areas where secondary growth can be expected, which is believed to account for the results.

Keyword
deforestation, water yield, hydrological models, Thailand, tropical forests
National Category
Social Sciences
Identifiers
urn:nbn:se:liu:diva-49107 (URN)10.1002/hyp.229 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-12Bibliographically approved
3. Calibrating a rainfall-runoff model for a catchment with limited data
Open this publication in new window or tab >>Calibrating a rainfall-runoff model for a catchment with limited data
2002 (English)In: Hydrological Sciences Journal, ISSN 0262-6667, E-ISSN 2150-3435, Vol. 47, no 1, p. 3-17Article in journal (Refereed) Published
Abstract [en]

A rainfall-runoff model has been established to simulate streamflow in a regulated catchment in southern India, where data were limited in relation to the basin's complexity. Within the basin is a network of hydropower reservoirs and tunnels that complicate the relationships between observed and natural flows. The basin is affected by two monsoons that dominate in different areas and can only be quantified through a relatively sparse raingauge network. These characteristics combine to make it difficult to satisfactorily define the spatial distribution of rainfall inputs to the basin. After critically assessing the data that were found to be inconsistent and unrepresentative, various assumptions about the operation of the system were tested. Despite incomplete streamflow data and the complex hydropower system, the limiting factor affecting successful simulations of streamflow at the basin outlet was the uncertain representativeness of the calculated areal rainfall. The final outcome is a model, which despite shortcomings, is considered to be a useful water resources management tool that provides a sound basis for further studies.

Abstract [fr]

Un modèle pluie-débit a été établi pour simuler le débit dans un bassin versant régulé dans le sud de l'Inde, où les données sont limitées par rapport à la complexité du bassin. Il existe au sein du bassin un réseau de réservoirs hydroélectriques et de tunnels qui complique les relations entre les débits observés et naturels. Le bassin est affecté par deux moussons qui dominent dans des régions différentes et qui peuvent être quantifiées uniquement grâce à un réseau de pluviomètres relativement clairsemé. La combinaison de ces caractéristiques rend l'observation de la distribution spatiale des apports pluviométriques difficile. Après avoir évalué les données et montré qu'elles sont incohérentes et non représentatives, nous avons testé plusieurs hypothèses sur le fonctionnement du système. Malgré des données de débit incomplètes et la complexité du réseau hydro-électrique, le facteur le plus limitant dans les simulations de débit a été la mauvaise représentativité des calculs de pluie moyenne. Le résultat final est un modèle qui, malgré ses défauts, est vu comme un outil utile de gestion des ressources en eau, qui fournit une base solide pour des études supplémentaires.

Keyword
rainfall-runoff modelling, Pitman model, limited data, land-use change, India
National Category
Social Sciences Interdisciplinary
Identifiers
urn:nbn:se:liu:diva-31766 (URN)10.1080/02626660209492903 (DOI)17590 (Local ID)17590 (Archive number)17590 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2018-01-13Bibliographically approved
4. Simulating the impacts of land-use and climate change on water resource availability for a large south Indian catchment
Open this publication in new window or tab >>Simulating the impacts of land-use and climate change on water resource availability for a large south Indian catchment
2002 (English)In: Hydrological Sciences Journal, ISSN 0262-6667, E-ISSN 2150-3435, Vol. 47, no 1, p. 19-30Article in journal (Refereed) Published
Abstract [en]

A monthly rainfall-runoff model was calibrated for a large tropical catchment in southern India. Various land-use and climatic change scenarios were tested to assess their effects on mean annual runoff and assured water yield at the Bhavanisagar Reservoir in Tamil Nadu, India. The largest increase in runoff (19%) came from converting forest and savanna (the indigenous control scenario) to agriculture. Mean annual runoff decreased by 35% after conversion to commercial forest and 6% after partial conversion to tea plantations. The predicted climate scenarios of reduced dry season rainfall decreased the annual runoff by 5% while enhanced annual rainfall caused a 17% increase in runoff. Even if land-use and climate changes had relatively large effects on runoff, the changes in reservoir yield which can be assured every year, were often less severe. This was probably due to the buffering effect of the reservoir and variation in the mean annual runoff.

Abstract [fr]

Un modèle pluie-débit mensuel a été calé pour un grand bassin versant tropical du sud de l'Inde. Différents scénarios de changement du climat et de l'occupation du sol ont été testés pour estimer leurs effets sur l'écoulement annuel moyen et sur le bilan du barrage Bhavanisagar, au Tamil Nadu, en Inde. La plus grande augmentation d'écoulement (19%) est obtenue dans le cas de la conversion de la forêt et de la savane (le scénario de contrôle indigène) en agriculture. L'écoulement annuel moyen décroît de 35% après conversion en forêts plantées, et de 6% après conversion partielle en plantations de thé. Le scénario climatique de réduction de la pluviosité en saison sèche réduit l'écoulement de 5%, tandis que celui de l'augmentation de la pluviosité annuelle engendre une augmentation de l'écoulement de 17%. Même si les changements de climat et d'occupation du sol ont des effets relativement importants sur l'écoulement, les changements dans la disponibilité annuelle du barrage sont souvent moins sévères. Cela est probablement dû à l'effet d'amortissement du stockage dans le réservoir.

Keyword
land-use change, climate change, hydrology, water resources, runoff, reservoir yield, India
National Category
Social Sciences
Identifiers
urn:nbn:se:liu:diva-48979 (URN)10.1080/02626660209492904 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-12Bibliographically approved
5. Local perceptions about forests and water in two tropical catchments
Open this publication in new window or tab >>Local perceptions about forests and water in two tropical catchments
2000 (English)In: GeoJournal, ISSN 0343-2521, E-ISSN 1572-9893, Vol. 50, no 4, p. 339-347Article in journal (Refereed) Published
Abstract [en]

This study was performed to assess how local inhabitants in two tropical watersheds value forests and perceive both forests and their own capacity to influence the hydrological cycle. Both service and productivity functions were strongly valued, particularly forests' rain-bringing capacity. The view that forests are directly responsible for increased precipitation was especially strong in low rainfall areas. Forests were also seen as important for their ability to retain soil water. The human activity most often mentioned as affecting water availability was tree planting while water conservation structures were not seen as detrimentally affecting others. Water was pictured very strongly as being part of a cycle so that which is used by humans or trees is not considered lost but only displaced to return again as rain.

Keyword
forest, local perceptions, water
National Category
Social Sciences Interdisciplinary
Identifiers
urn:nbn:se:liu:diva-31763 (URN)10.1023/A:1011012420808 (DOI)17587 (Local ID)17587 (Archive number)17587 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2018-01-13Bibliographically approved

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