An application of NAM model for runoff volume estimation in Chi basin.

Hormwichian, Rattana ; Homdee, Tipaporn ; Kangrang, Anongrit 等

1. INTRODUCTION

Chi basin is located in the northeast of Thailand, having total area of 49,477 square kilometers or 9.66% of total area of the country. At present, average amount of runoff in Chi basin flowing to Mun River is about 8,752million cubic meters per year. Since majority of people in the area are farmers, they need many water sources for agriculture, so the hydraulic model is required to be improved always. But in some area, there is no runoff gauging station. So, the model was conducted in order to estimate runoff volume of the basin before flowing to the main stream of Chi river. The running of the sub-basin flow to the main stream is lateral flow of the main stream used for river simulation. If the accurate data can be estimated, it will make planning for water management or flood warning better.

Examples of models that are well known and accepted in many countries, including the Sacramento model (Burnash, 1995), HBV model (Bergstrom, 1995) and MIKE11/NAM model (Havno et al., 1995). In this research model, the NAM is part of MIKE11 program developed by Danish Hydraulic Institute (DHI) for calculating runoff volume. The concept of this model is based on physical structure and Semi Empirical equation. The main parameter for each sub-basin consisted of 9 parameters, i.e. the upper limit of moisture content in the surface zone (Umax), the upper limit of moisture content in the root zone (Lmax), the overland flow runoff coefficient (CQOF), Time constant for Interflow (CKIF), time constant for interflow and overland flow routing (CK1,2), the threshold value for overland flow (TOF), the root zone threshold value for interflow (TIF), the root zone threshold value for groundwater recharge (TG), and the time constant for base flow (CKBF). Each parameter has relationship in form of hydrologic cycle employing estimation derived from calibration with actual data. And in this model, there was repeatedly data processing with several conditions until the appropriated data was found. This is the daily runoff amount. When the parameter of basin which represents the area in Chi basin was obtained, the runoff amount of sub-basin within Chi basin also can be estimated.

2. SCOPE OF THE STUDY

2.1 This study used drainage area of station E29 at Ban Pha Nok Khao, Loei province; station E65 at Ban Tha Hai, Udon Thani province, and Station E68A at Ban Khong Po, Nong Bua Lamphu to represent sub-basins of Chi basin (Fig. 1).

2.2 In this study, a control structures in channel is not considered.

[FIGURE 1 OMITTED]

3. MATHEMATICAL MODEL

The hydrologic model employed in this study is the NAM model developed by the Department of Hydrodynamics and Water Resources of the Technical University of Demark. It is the lumped conceptual model capable of simulating various components of catchment hydrology, which consist of three storage components of the hydrologic cycle.

3.1 Surface Zone

Moisture intercepted on the vegetation is stored as well as water trap in depressions and in the uppermost, cultivated part of the ground is represented as surface storage. [U.sub.max] is the maximum upper zone storage of water in the surface storage.

The upper zone storage (U) in the surface storage is continuously reduced by evaporative consumption as well as by interflow. When the upper zone storage exceeds the maximum surface storage, will enter the streams as overland flow, wherefore the remainder is diverted as infiltration into the root zone and groundwater storage.

3.2 Root Zone

The soil moisture in the root zone, a soil layer below the surface from which the vegetation can draw water for transpiration, is represented as root zone storage. [L.sub.max] is the maximum root zone storage of the amount of water in root zone storage.

3.3 Overland flow

From a structural model, when the surface storage spills, i.e. when U>[U.sub.max], the excess water gives rise to overland flow as to infiltration. QOF is the part of the excess water that contributes to overland flow. It is assumed to be proportional to the excess water and to vary linearly with the relative soil moisture content (L/[L.sub.max]), of the root zone storage, the following equation:

[Q.sub.OF] = CQOF(L/[L.sub.max]) - TOF/ 1 - TOF For L/[L.sub.max] > TOF (1)

[Q.sub.OF] = 0 For L/[L.sub.max] [less than or equal to] TOF (2)

When CQOF is the overland flow runoff coefficient

TOF is the threshold value for overland flow

3.4 Base flow

The base flow from the groundwater storage is calculated as the outflow from a linear reservoir with time constant (CKBF). The base flow (BF) is given by

BF = ([GWL.sub.BF0] - GWL)[S.sub.Y] [(CKBF).sup.-1] For GWL [less than or equal to] [GWL.sub.BF0] (3)

BF = 0 For GWL > [GWL.sub.BF0] (4)

When [S.sub.Y] is specific yield of reservoirs

GWL is groundwater table depth

[GWL.sub.BF0] is maximum groundwater table depth by base flow

4. AN APPLICATION AND RESULTS OF MODEL

NAM model is a mathematical model based on lumped conceptual model of hydrological process, employing parameters and variables to simulate relationship between rainfall and runoff continuously, with the following procedures.

1. To collect data of daily rainfall, daily evaporation, daily runoff, and geography information system.

2. To synthesis the missing data

3. To divide the area of sub-basin by considering contour line of Chi basin area.

4. To calculate average rainfall of each catchments area.

5. To use NAM Model to determine a set of parameters by calibrating with the actual runoff measured at that point; station E29 at Ban Pha Nok Khao, Loei province, station E65 at Ban Tha Hai, Udon Thani province, and Station E68A at Ban Khong Po, Nong Bua Lamphu province, by using daily data from year 2001-2005.

6. To verify the set of parameters derived from calibrating the model with other events of raining by using daily data from year 2006-2007.

From study on application of model to the catchments area of those 3 stations of Chi basin such as station E29 at Ban Pha Nok Khao, Loei province, station E65 at Ban Tha Hai, Udon Thani province, and Station E68A at Ban Khong Po, Nong Bua Lamphu province, it was found that the hydrograph derived from calibrating the model had the same trend as the behavior of occurrence of actual runoff of the catchments area (Fig. 2). The parameter derived from calibration is shown on table 1 and table 2.

[FIGURE 2 OMITTED]

The result of model verification by employing the set of parameters and data from year 2006-2007, the correlation coefficient were in range of 0.7-0.738, which were acceptable ranges.

5. CONCLUSION

When applying NAM Model to determine runoff amount from daily rainfall and daily evaporation rate, a significant part of the model was adjusted i.e. parameters directly correlating with rainfall and flow behavior in the stream to close to actual condition as much as possible. From calibration, the derived correlation coefficient was in range of 0.710-0.824 and from verifying the data in year 2006-2007, the correlation coefficient was in range of 0.7-0.738, which were acceptable ranges. So, the parameters mentioned depends on land use and human activity which influenced the change of flow conditions. However, application of the model would be the best when it is applied to the natural flood routing. In this study, it is recommended that the impact from changing land use and activities on the catchments and the impact of control structures in channel should be comprehensively studies in the future.

6. ACKNOWLEDGEMENTS

Without the financial support by the faculty of Engineering, Mahasarakham University, this research would not have been possible. We would like to thank Mekhala Center, Department of Water Resources, Thailand, for technical support of Mike11/NAM program.

7. REFERENCES

Bergstrom, S. (1995). The HBV model, In: Computer Models of Watershed Hydrology, Singh, V.P., (Ed.), 443-476, Water Resources Publications, ISBN 0-918334-91-8 Highlands ranch, Colorado, U.S.A.

Burnash, R.J.C. (1995). The NWS river forecast system-catchment model, In: Computer Models of Watershed Hydrology, Singh, V.P., (Ed.), 311-366, Water Resources Publications, ISBN 0-918334-91-8 Highlands ranch, Colorado, U.S.A.

Havno, K.; Madsen, M.N. & Dorge, J. (1995). MIKE11-A generalized river modeling package, In: Computer Models of Watershed Hydrology, Singh, V.P., (Ed.), 733-782, Water Resources Publications, ISBN 0-918334-91-8 Highlands ranch, Colorado, U.S.A.

Kobold, M. & Brilly, M. (2006). The use of HBV model for flash flood forecasting. Natural Hazards and Earth System Sciences, Sci., Vol.6, No.3, 407-417, ISSN 1561-8633

Tab. 1. Parameter calibration results ([U.sub.max], [L.sub.max],
CQOF, CKIF, and [CK.sub.1/2])

Parameters     [U.sub.max]      [L.sub.max]          CQOF

E29                 20              300              0.55
E65                 10              159             0.445
E68A                20              300             0.269

Parameters         CKIF         [CK.sub.1,2]

E29                1000              32
E65                916               48
E68A               500               48

Tab. 2. Parameter calibration results (TOF, TIF, TG, and
CKBF)

Parameters       TOF         TIF         TG         CKBF

E29             0.907       0.424       0.720       4000
E65             0.968       0.084       0.136       1114
E68A            0.208       0.595       0.806       1000