Simulations of the Effect of Deforestation on Surface Water Runoff and Flooding in the Swift River Watershed. INTRODUCTION Background Intense rainfall associated with Tropical Storm/Hurricane Michelle in October and November 2001 resulted in severe flooding in the parishes of Portland and St. Mary. The floods were most devastating to communities in the Swift and Spanish River Watersheds where floodwater depth of more than 3m were reached. Damage to infrastructure and housing forced the evacuation of some communities and cut off access to others. An impact assessment conducted after the flood identified several factors that may have contributed to the high runoffs. One of the factors identified by the document was anthropogenic activity in the upper watersheds. Anthropogenic activities effectively reduce the forest canopy in these areas and expose the soil to direct impact by precipitation, which increases surface runoff. Studies on forest depletion in Jamaica, from the early 1960s to mid 1990s, have estimated that annual deforestation range between 0.1 and 11.3%1. More recently (in March 2000), the Forestry Department and the Trees for Tomorrow Project carried out an analysis of forest cover change over the period 1989 to 1998. The study showed that there was a loss in forest land amounting to 30.6km2 (3063.6ha) or 0.91% of the forest land in 1989, which translates to an annual loss of approximately 0.1%1. While many studies have been conducted on deforestation in Jamaica, very few have sought to quantify its impact on surface runoff and flooding. This may have been as a result of the tedious and complicated procedures that were involved in such computations. Current technologies have significantly reduced these complex procedures so that computations of this nature are much easier and quicker to perform. The Watershed Modeling System software developed by Brigham Young University, in tandem with ArcView GIS were used in this study to simulate surface runoff for different types of forest cover over the Swift River Watershed.
Objective The objectives of the project are: 1) To quantitatively explore the extent to which deforestation may have impacted surface runoff. 2) To conduct a comparative analysis of the impact of further deforestation vis-àvis reforestation on surface runoff. 1 Forestry Department,.National Forest Management and Conservation Plan, March 2001, pg. 23-25. 2 Scope of Work This paper is focused on surface runoff within the Swift River Watershed and will include: 1) Analysis of the land use/forest cover over the watershed. 2) Computer simulation of surface runoffs for 5, 50, and 100yr rainfall events for various land use changes. Swift River Watershed Hydrology The Swift River Watershed forms a part of the Blue Mountain North Basin, which is found in the northeastern section of the island (Map 1). This part of the island records an average annual rainfall of approximately 5,024mm[2]. The Swift River is approximately 100 square kilometers[3] and is drained by the Swift River and its network of tributaries, mainly the Bantimore and Back Rivers. 2 Louis Berger Intl. Inc., Master Plan Report, Port Antonio Sanitation Study. Environmental Assessment, 1996, pg. VIII-11 3 Water Resources Authority Switf river watershed Other Basins Blue monutain north basin 50 0 50 Kilometers N Map 1: Location of the Swift River Watershed 3
Hydrogeology The Swift River Watershed is characterized by the dominance of rock belonging to the basement aquiclude, which are considered to be impermeable rocks. Being impermeable, there is no storage of water within these rocks and rainfall is transmitted very quickly to surface runoff. Physiography The Swift River Watershed is in general mountainous and densely vegetated. The upper reaches tend to be very steep and are characterized by a predominantly northerly direction towards the north coast where they enter the numerous waterfalls both in the main channels and side gullies. The rivers downcut through the bedrock to form.v. shaped valleys with steep side slopes. The middle to lower reaches are generally flatter and is where the majority of the affected communities are located. Methodology Land Use Change and Surface Runoff Analysis The present land use for the Swift River Watershed (Map 2) was obtained from the Forestry Department.s.National Forest Management and Conservation Plan., March 2001. The definitions are presented in Table 1. Deforestation was assumed to have taken place in the mixed and non-forest classifications. These classifications refer to forested areas where anthropogenic activities have taken place4. The disturbed broadleaf sub-class was also considered to have undergone some level of deforestation despite it being regarded as a natural forest. Table 1 Definition of Land Use and Forest Cover. LAND USE / COVER Class Sub-class DEFINITION Closed Broadleaf (PF) Closed primary forest with broadleaf trees at least 5m tall and crowns interlocking, with minimal human disturbance. Disturbed Broadleaf (SF) Disturbed Broadleaf Forest with broadleaf trees at least 5m tall and species-indicators of disturbance such as Cecropia peltata (trumpet tree).
Forest Bamboo (BB) Bambusa vulgaris (Bamboo brakes) on the lower shale hills (disturbed forest) Disturbed Broadleaf Forest and Fields (SC) >50% Disturbed Broadleaf forest; >25% fields Bamboo and Disturbed Broadleaf Forest (BF) >50% Bamboo; >25% Disturbed Broadleaf forest Bamboo and Fields (BC) >50% Bamboo; >25% fields Mixed Fields and Disturbed Broadleaf Forests (CS) >50% fields; >25% Disturbed Broadleaf forest Plantations (PC) Tree crops, shrub crops like sugarcane, bananas, citrus and coconuts. Fields (FC) Herbaceous crops, fallow, cultivated grass/legumes Non- Forest Buildings and Other Infrastructure (BA) Buildings and other constructed features such as airstrips, quarries etc. Source: Forestry Department,.National Forest Management and Conservation Plan., March 2001 The ArcView GIS software was used to create shapefiles of various combinations of forest cover over the Swift River Watershed by interchanging the different subclass classifications. The changes in the forest cover were regarded as either deforestation or reforestation depending on the definition of the sub-classes (Table 1) that were being interchanged. These land use shapefiles together with a soiltype map and a digital elevation model (DEM) of the Swift River Watershed were exported to the WMS software where the HEC-1 hydrologic modeling system developed by the US Army Corp of Engineers was used to simulate the flood hydrographs. The simulation of the flood hydrographs was based on the US Soil Conservation Service (SCS) Rainfall- Runoff Relation. The land use change scenarios for deforestation and reforestation are presented in Tables 2 and 3 respectively. The impact of deforestation on surface runoff in the watershed was evaluated by first comparing the simulated results of the present land use with that of a closed broadleaf forest for the entire watershed, having restored all other subclasses with the closed broadleaf forest. By making this assumption, the maximum possible increase in runoff to the present land use was determined. Secondly, the simulated result of the present land use was compared with the results of various combinations of land use that implied deforestation of varying degrees. Varying one sub-class
while holding the others constant measured the extent to which different degrees of deforestation will impact runoff. 5 The impact of reforestation was evaluated by comparing the simulated results of the present land use with the results of various combinations of land use that implied reforestation of different degrees. Table 2 Land Use Change Scenarios for Deforestation LAND USE CHANGE From: T0: Present Land Use Bamboo and Fields (BC) Plantations (PC) Disturbed Broadleaf Forest and Fields (SC) Fields and Disturbed Broadleaf Forests (CS) Closed Broadleaf Forest (PF) Disturbed Broadleaf (SF) Fields and Disturbed Broadleaf Forests, Disturbed Broadleaf Forest and Fields and Disturbed Broadleaf (CSS) Plantations (PC) Disturbed Broadleaf Forest and Fields (SC) Fields and Disturbed Broadleaf Forests (CS) Disturbed Broadleaf (SF) Fields and Disturbed Broadleaf Forests (CS) Disturbed Broadleaf (SF) Disturbed Broadleaf Forest and Fields (SC) Table 3 Land Use Change Scenarios for Reforestation LAND USE CHANGE From: T0: Present Land Use Bamboo and Fields (BC) Disturbed Broadleaf Forest and Fields (SC) Fields and Disturbed Broadleaf Forests (CS) Disturbed Broadleaf (SF) Fields and Disturbed Broadleaf Forests, Disturbed Broadleaf Forest and Fields and Disturbed Broadleaf (CSS) Closed Broadleaf Forest (PF) Fields and Disturbed Broadleaf Forests (CS) Disturbed Broadleaf Forest and Fields (SC) Fields and Disturbed Broadleaf Forests (CS) Disturbed Broadleaf (SF)
Disturbed Broadleaf Forest and Fields (SC) Disturbed Broadleaf (SF) 6 RESULTS Deforestation Present Land Use vs. Closed broadleaf Forest The maximum possible increase in surface runoff as a result of deforestation within the Swift River Watershed to its current level for the three return period rainfall events modeled ranged from 18% for the 100yr event to 38% for the 5yr event. The percentage increase in runoff in the upper watershed for all three events is higher than the percentage increase for the entire watershed combined. As the return period rainfall events decrease, the percentage increase in runoff increases for both the upper watershed and the entire watershed combined (Table 4). Table 4 Comparison of the Peak Surface Runoffs for the Present Land Use and the Closed Broadleaf Forest Cover over the entire Swift River Watershed. Peak Surface Runoff (m3/s) Return Period Watershed Closed Broadleaf Forest Present Land Use Difference % Increase Upper 897 1,116 219 24% 100yr 1,972 2,335 363 18% Upper 717 920 202 28% 50yr 1,640 1,984 343 21% Upper 195 309 114 58% Swift River Watershed 5yr 616 851 235 38% Present Land Use vs. Further Deforestation
Further deforestation of the Swift River Watershed from its present land use could result in increased surface runoff of between 2 and 55% in the upper watershed for the 50 and 100yr return period rainfall events and more than 100% percent for the 5yr return period rainfall event. The percentage increase in runoff for the entire watershed is approximately half the percentage increase in the upper watershed for the three rainfall events but the magnitude of runoff is approximately the same (Table 5). Runoff in the upper watershed for the 100 and 50yr rainfall events will increase by 48 to 55% if the closed broadleaf forest cover (PF) is replaced with fields and disturbed broadleaf (CS) which is >50% fields >25% disturbed broadleaf or by Plantations (PC). For the 5yr event, runoff in the upper watershed will increase by approximately 120%. On the other hand if the closed broadleaf forest cover is replaced with disturbed broadleaf and fields (SC) which is >50% disturbed broadleaf >25% fields, runoff in the upper watershed will increase by 26 to 30% for the 100 and 50yr rainfall events and by 60% for the 5yr rainfall event. The increase in runoff for other land use changes ranged from 2 to 38% for both the upper watershed and the entire watershed combined for all three rainfall events. 7 Table 5 Impact of Different Degrees of Deforestation of Surface Runoff Increase in Surface Runoff (m3/s) Land Use Change PF-BC PF-PC PF-SC PF-CS PF-SF CSS-PC SC-CS SF-CS SF-SC Return Period Watershed Present Land Use 1,116 168 531 294 531 168 195 121 81 25 Upper 15% 48% 26% 48% 15% 18% 11% 7% 2% 2,335 170 546 298 546 170 266 158 124 40 100yr 7% 23% 13% 23% 7% 11% 7% 5% 2% 920 155 507 275 507 155 181 112 75 23 Upper 17% 55% 30% 55% 17% 20% 12% 8% 2% 1,984 158 521 280 521 158 255 150 120 38 50yr
8% 26% 14% 26% 8% 13% 8% 6% 2% 309 101 367 184 367 101 118 72 47 14 Upper 33% 119% 60% 119% 33% 38% 23% 15% 5% 851 100 372 184 372 100 193 107 92 29 5yr 12% 44% 22% 44% 12% 23% 13% 11% 3% Reforestation The impact of reforestation on surface runoff was very marginal, when compared to the impact of deforestation, showing decreases in runoff of between 1 and 18% for the 50 and 100yr rainfall events and between 2 and 31% for the 5yr return period rainfall event. The more significant reductions in runoffs were as a result of the conversion of large areas of mixed fields and disturbed broadleaf forest to closed broadleaf forest. Between 60 and 75% of the land use change resulted in a reduction in surface runoff ranging from 2 to 5% for both the upper watershed and the entire watershed combined (Table 6). Table 6 Impact of Different Degrees of Reforestation of Surface Runoff Decrease in Surface Runoff (m3/s) Land Use Change BC-PF CS-PF SC-PF SF-PF CSS-PF CS-SC CS-SF SC-SF Return Period Watershed Present Land Use 1,116-25 -23-120 -28-177 -12-17 -53 Upper -2% -2% -11% -3% -16% -1% -2% -5% 2,335-38 -51-160 -44-268 -26-38 -72 100yr -2% -2% -7% -2% -11% -1% -2% -3% 920-24 -22-111 -26-164 -11-16 -49 Upper -3% -2% -12% -3% -18% -1% -2% -5% 1,984-36 -49-150 -42-253 -25-37 -68 50yr -2% -2% -8% -2% -13% -1% -2% -3% 309-14 -13-65 -16-94 -7-10 -30 Upper -5% -4% -21% -5% -31% -2% -3% -10% 851-25 -40-99 -31-172 -21-30 -47 5yr -3% -5% -12% -4% -20% -2% -4% -5% 8 DISCUSSION The results of the simulations indicate that the deforestation of the watershed has a
far greater impact on surface runoff than reforestation. The primary reason for this is that approximately 85% of the watershed is comprised of closed broadleaf forest and disturbed broadleaf forest mixed with fields while plantations, bamboo and fields, and built up areas comprised the other 15%. The densely vegetated watershed means that larger areas of land were subjected to deforestation in the simulation exercise hence higher increases in runoff. In addition, the forested areas are located mainly in the upper watershed where the slopes are much steeper and runoff occurs much quicker and with greater velocity so that deforestation in these areas increases the runoff significantly. This was demonstrated in the high percentage increases in the runoff from the upper watershed for the 5yr rainfall event when compared to the increases for the entire watershed. The less intensity rainfall allows greater infiltration of the water into the soil in the flat areas while the effect of the steep slopes in the upper watershed allows less infiltration and quicker runoff. Additional research to develop an index of runoff per unit area for each land use/forest cover is recommended so that a better comparison of their impact can be made. Mapping of the flood boundaries to determine the extent to which increased runoff, as a result of deforestation, will affect communities is also recommended. BIBLIOGRAPHY 1. Forestry Department,.National Forest Management and Conservation Plan., March 2001 2. WRA,.Rapid Impact Assessment., April 2002 3. Louis Berger Intl. Inc., Master Plan Report, Port Antonio Sanitation Study. Environmental Assessment, 1996, pg. VIII-11 Author: Lawrence Barrett, - larrosim_b@hotmail.com Co-Authors: Donald Hardware, Andreas Haiduk Address: c/o Water Resources Authority, Hope Gardens, P.O. Box 91, Kingston 7, Jamaica, Telephone: (876) 927-0077 Fax: (876) 977-0179 e-mail : wra@colis.com