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Hydrological impacts of forest conversion to agriculture in a large river basin in northeast Thailand
Linköping University, The Tema Institute, Department of Water and Environmental Studies. Linköping University, Faculty of Arts and Sciences.
Linköping University, The Tema Institute, Department of Water and Environmental Studies. Linköping University, Faculty of Arts and Sciences.
Department of Agricultural Engineering, Khon Kaen University, Thailand.
2001 (English)In: Hydrological Processes, ISSN 0885-6087, E-ISSN 1099-1085, Vol. 15, no 14, 2729-2748 p.Article 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.

Place, publisher, year, edition, pages
2001. Vol. 15, no 14, 2729-2748 p.
Keyword [en]
deforestation, water yield, hydrological models, Thailand, tropical forests
National Category
Social Sciences
URN: urn:nbn:se:liu:diva-49107DOI: 10.1002/hyp.229OAI: diva2:270003
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2012-07-09Bibliographically approved
In thesis
1. Do forests have an impact on water availability?: Assessing the effects of heterogeneous land use on streamflow in two monsoonal river basins
Open this publication in new window or tab >>Do forests have an impact on water availability?: Assessing the effects of heterogeneous land use on streamflow in two monsoonal river basins
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. 79 p.
Linköping Studies in Arts and Science, ISSN 0282-9800 ; 222
National Category
Social Sciences Interdisciplinary
urn:nbn:se:liu:diva-31783 (URN)17607 (Local ID)91-7219-882-6 (ISBN)17607 (Archive number)17607 (OAI)
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: 2012-07-09Bibliographically approved

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