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Impacts of Land Use Changes on Hydrological Regime - A Case Study of Randenigala and Kotmale Catchments in Sri Lanka
Methodology and Results
- Hydrological Data
Monthly incremental inflow data were collected for the period of 1950 to 1996 to investigate the water yield and yielding patterns. Prior to the reservoir construction (before 1985), stream flow measurements were available and after reservoir construction (after 1985), water yield data were obtained from reservoir water balance.
Monthly rainfall data were also collected for a period of 40 years for 14 stations at Kotmale and 7 stations at Randenigala. These rainfall records were also separated into two periods i.e. before and after the AMDP. In addition, data for every decade were analyzed separately.
For each period, the river flow and rainfall data series were divided into four seasons; namely North East monsoon (NEM, Dec-Feb), First Inter-monsoon (FIM, Mar-Apr), South West monsoon (SWM, May-Sep) and Second Inter-monsoon (SIM, Oct-Nov). Areal rainfall for each subcatchment for each season was calculated based on the coverage of Thyestean polygon. In addition, dry weather flow and wet weather flow were analyzed separately with corresponding areal rainfall data for these periods.
- Time Series Analysis
Trends and Mean shifts of rainfall and river flow time series were examined through a comprehensive time series analysis. Linear rend analysis revealed that there are no linear trends in the hydrological time series although in few occasions, outstanding deviations from stationary was manifested in monthly time series. These deviations were observed for both periods before and after AMDP and hence, no statistical inference can be made on these deviations. Mann-Whitney test was employed to find the presence of mean shifts in the data series. Again, no consistent shift in the mean is displayed by any of the time series; however, data series of some of the decades show marginal deviations. These variations could be attributed to isolated cases of measurement errors or faulty observations, which were commonly experienced during the data collection process.
- Detection of Land Use Change
The first land use inventory of Sri Lanka has been recorded in 1956-1961 period at the scale of 1:63,360 based on aerial photographs acquired in 1956 at 1:40,000 (Chandrasekera, 2000). These land use and cover data for 1956 were available from the Hunting Survey maps, which were digitized to Workstation Arc/Info version 7.12. In addition, TIN module and GRID modules of Arc/Info software was used to identify sub catchment boundaries from the digitized contours and spot heights. .
Present land use and cover status was identified through the classification of IRS LISS II imagery acquired in 1992 and air photos taken between 1992 and 1995. The scale of the map is 1; 10,000 and the land use/ cover was identified based on 10 broad classes. These classes were determined considering the hydrological importance and hydrological response of each category.
Because of the differences in the definition of classes in 1956 and 1992 maps, reclassification of the classes were required to bring these two maps to a comparable level. Further, due to the different mapping scales, elimination technique was used to remove areas smaller than the minimum mapping parcel size. Based on the 1956 map, minimum mapping parcel size was determined as 1 ha.
The scale of the changes of land use and cover categories were assessed and change detection maps for each and every land use category were prepared in GIS. The generalized spatial distribution of land use changes in Randenigala and Kotmale sub catchments are shown in Figure 02 and Figure 03, respectively. The land use change statistics are given in Table 01 and Table 02 for Randenigala and Kotmale, respectively.
Table 01. Land Use Changes from1956 - 1992 in Randenigala Sub Catchment
| Land Use |
1956 |
|
1992 |
|
Change |
| |
Extent ha |
% |
Extent ha |
% |
% |
| Tea |
6638.36 |
14.82 |
3933.28 |
8.78 |
-6.04 |
| Other Perennials |
2749.17 |
6.14 |
2896.9 |
6.46 |
0.32 |
| Paddy |
3756.06 |
8.39 |
3423.17 |
7.64 |
-0.75 |
| Annual Crops |
15024.9 |
33.55 |
8184.63 |
18.30 |
-15.25 |
| Grassland |
3318.91 |
7.42 |
1119.13 |
2.50 |
-4.92 |
| Scrubland |
77.95 |
0.17 |
4111.58 |
9.18 |
9.01 |
| Forest Plantations |
342.02 |
0.76 |
2221.74 |
4.95 |
4.19 |
| Natural Forest |
12511 |
27.94 |
14812.9 |
33.10 |
5.16 |
| Urban/Unproductive |
50.04 |
0.11 |
1329.87 |
2.95 |
2.84 |
| Water Bodies |
315.17 |
0.70 |
2750.71 |
6.14 |
5.44 |
| Total |
44783.9 |
100 |
44783.9 |
100 |
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Table 02. Land Use Changes from1956 - 1992 in Kotmale Sub Catchment
| Land Use |
1956 |
|
1992 |
|
Change |
| |
Extent ha |
% |
Extent ha |
% |
% |
| Tea |
32108.9 |
56.21 |
24955.6 |
43.69 |
-12.52 |
| Other Perennials |
1959.06 |
3.43 |
2072.32 | 3.63 |
0.20 |
| Paddy |
1164.35 |
2.04 |
806.17 |
1.41 |
-0.63 |
| Annual Crops |
191.45 |
0.34 |
2040.19 |
3.57 |
3.23 |
| Grassland |
2506.33 |
4.39 |
3628.06 |
6.35 |
1.97 |
| Scrubland |
35.33 |
0.06 |
1950.97 |
3.42 |
3.35 |
| Forest Plantations |
1960.6 |
3.43 |
4079.67 |
7.14 |
3.71 |
| Natural Forest |
16531.6 |
28.94 |
14610.9 |
25.58 |
-3.36 |
| Urban/Unproductive |
388.69 |
0.68 |
1864.8 |
3.26 |
2.58 |
| Water Bodies |
273.79 |
0.48 |
1111.35 |
1.95 |
1.47 |
| Total |
57120.1 |
100 |
57120.1 |
100 |
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