P B Hunukumbura1 S B Weerakoon1 Application of physically based hydrological model to the Upper Kotmale basin P B Hunukumbura1 S B Weerakoon1 University of Peradeniya, Sri Lanka 20th December 2004 – SLAAS , Colombo

Objective of study SLAAS - 2004 Develop a physically based hydrological model for stream flow prediction in the Upper Kotmale basin Compare the performance of the Similar Hydrological Element Response model (SHER) with different types of basin distribution

Introduction Water Resources in Sri Lanka Rainfall Surface water SLAAS - 2004 Water Resources in Sri Lanka Rainfall Southwest monsoon – May to Sep. Northeast monsoon – Dec. to Feb. Inter monsoon – Mar.to Apr & Oct. to Nov. Surface water Sri Lanka has 103 river basins and their sizes vary from 10 km2 to 10450 km2

Study area Basin area = 304 km2 Elevation = 1200m to 2500m SLAAS - 2004 Study area Basin area = 304 km2 Elevation = 1200m to 2500m Annual rainfall = 2200 mm to 2600 mm The Upper Kotmale Basin

Land use map Tea 44% Forest 36% Built up Land 7%| Grass 5% SLAAS - 2004 Tea 44% Forest 36% Built up Land 7%| Grass 5% Cash Crops 5%

Preparation of GIS maps Slope map SLAAS - 2004 Preparation of GIS maps The area and slope of each block was estimated by using a GIS database. The 1: 50000 scale digital maps prepared by the Survey Department were used in this study. A TIN model was first created by using the contour map and then the elevation GRID was created using the TIN model and corrected using the stream network. The slope grid of the basin was then derived.

Soil type map SLAAS - 2004

The SHER model SLAAS - 2004 The Similar Hydrological Element Response model (SHER) is a lumped physically based model developed in the Extend simulation environment incorporating the physical processors to represent basin heterogeneity The basin is distributed by considering the basin stream network, soil distribution and slope variation.

Criteria for dividing the basin for SHER model application SLAAS - 2004 In this study, three models were developed and performance of each model predictions were compared SHER 1 The basin was divided into three regions based on three main soil series. Each region was modeled using soil moisture block.

SLAAS - 2004 Block diagram for SHER 1 Soil NE Basin Soil HOR Soil MAT

Cont.. SLAAS - 2004 SHER 2 The basin was divided into three regions based on three main soil series. Each soil series was again subdivided into two slope groups

Block diagram for SHER 2 % of the area with Slope 0-20 >20 Block 3 SLAAS - 2004 Block diagram for SHER 2 % of the area with Slope 0-20 >20 Block 3 Block 1 Block 2 Block 6 Block 4 Block 5 Soil NE Basin Soil HOR Soil MAT

Cont.. SLAAS - 2004 SHER 3 The basin was divided into three regions based on three main soil series. Each region was divided to two sub regions. The area that lies within 0.5 km from perennial streams is taken as one sub region and the rest as the other region. The latter is subdivided into two regions according to the slopes

Block diagram for SHER 3 Block 6 Block 4 Block 5 Block 7 Block 8 SLAAS - 2004 Block diagram for SHER 3 Block 6 Block 4 Block 5 Block 7 Block 8 Block 2 Block 1 Block 3 % of the area with Slope 0-20 >20 Block 9 Soil NE Basin Soil HOR Soil MAT Other Near Stream Near Stream

Data Stream flow Evaporation Rainfall 1987 to 1993 SLAAS - 2004 Stream flow Daily Stream flow data at Thalawakele flow gauge was used Evaporation Daily pan evaporation data at Kotmale was used Rainfall Daily rainfall data at six stations were used 1987 to 1993

Model evaluation Calibration period - 1987 to 1988 SLAAS - 2004 Calibration period - 1987 to 1988 Verification period - 1989 to 1993 Two statistical indices were used to compare the model predictions Nash Sutcliffe coefficient, Root mean square error,

Results SLAAS - 2004 Calibration Period

Results Cont.. SLAAS - 2004 Verification Period

Simulated mean flow/(mm/day) 2.39 2.33 3.36 3.1 2.91 Parameter 1987 to 1988 - Calibration 1989 to 1993 – Verification SHER 1 SHER 2 SHER 3 SHER1 SHER2 SHER3 Nc 0.47 0.57 0.64 0.54 0.63 0.65 RMSE 1.84 1.66 1.53 2.82 2.53 2.46 Simulated mean flow/(mm/day) 2.39 2.33 3.36 3.1 2.91 Standard deviation of simulated flow 2.18 1.74 1.95 3.8 3.2 Observed mean flow/(mm/day) 2.85 Standard deviation observed flow 4.15

Conclusion SLAAS - 2004 Physically based model (SHER ) was developed and calibrated for the Upper Kotmale basin to predict the stream flow at the basin outlet. The third type SHER 3 where the basin was distributed considering the soil type, stream network, and the basin slope, gives the highest Nc value of 0.65 and the lowest RMSE value of 2.46 in the verification period. It is found that the Nc value increases by 0.1 when the SHER 1 distributed further corresponding to SHER 2 The accuracy of the model predictions increases with the distribution the basin according to slope

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