Application of Hydrologic Models for flood forecasting
The term Hydrology deals with water of earth, their presence, distribution, circulation in relation to living thing. This also deals with the relationship of water with environment within each phase of hydrologic cycle.
Water is foundation of life and plays a vital role in economy of country. The water cycle is characterized by the hydrological cycle. Pakistan is located in South Asia between 24˚37’N latitude and 66˚77’E. It occupies a total geographical area of 803,943 km2. High mountains ranges includes Himalaya, Karakoram Hindukush ranges are present in the north of the country.
Pakistan has more than 5000 snow reservoir and glaciers along with the second largest Siachen glacier. The fresh water is preserved in these reservoir during the winter season. Later on, feeds to the river in the summer season due to the gradual melting of ice and snow in upper parts of country. The Indus River run off is highly depends upon snow melt in these upper parts. More than 70% cultivated area lies along the Indus River. Due to deforestation, land cover change, irrigation, various changes have been occurred in hydrologic systems. Climate change, soil heterogeneity has direct impact on the discharges of many rivers in and around the world. Different hydrologic phenomena and hydrologic cycle are studied to find out these variations.
Now days, various hydrological models have been developed across the world. This will help to find out the impact of climate and soil properties on hydrology and water resources. Each model has its own unique characteristics. These models are classified based on model input and parameters. It also based on the extent of physical principles applied in the model.
HBV model (Hydrologiska Byrans Vattenavdelning model) originally developed by Sten Bergström at the Swedish Meteorological and Hydrological Institute (SMHI). It is one of the most used hydrological models in the world. This is semi distributed conceptual model. The entire catchment is divided into sub catchments, which are further divided into different elevation and vegetation zones. It runs on daily and monthly rainfall data, air temperature and evaporation. Air temperature data are used for calculating snow accumulation. HBV model main structure are shown in above figure.
The general water balance equation used is P-E-Q=d/dt (SP+ SM +UZ +LZ +lakes)
P is precipitation, E is evaporation, Q is runoff , SP is the snow pack. SM is the soil moisture, UZ and LZ are the upper and lower ground water zone and lakes represent the volume of lake. Different model versions are available and used in different countries with different climatic conditions. Degree day method is used to simulate snow accumulation and snow melt. Ground water recharge, runoff and actual evaporation are simulated as functions of actual water storage.
HBV-light is a new version of HBV model and it uses a warm-up period. In this version, the state variables will get its appropriate values as per meteorological data and parameter values. The HBV model can be used to extend runoff data series, water balance studies, runoff forecasting (flood warning and reservoir operation). It is also used to simulate discharge from catchments and to simulate climate change effects.
MAJOR FIELDS OF APPLICATION OF THE HBV MODEL
Firstly, the primary application of HBV model was hydrological forecasting. But now this field has broadened. It covers the today real-time forecasting, runoff record extensions, data quality control and design floods. It also tell us filing in gaps, studies related to water balance, synoptic water balance mapping and simulation of effects of ground water response and changing climate.
Real-time forecasting
Forecasts maybe short term (for few days) and long term (covering the entire snow melt season). Meteorological forecasts are frequently used a model input for short term forecasting. Long term forecasting is based on current hydrological conditions (as defined by model) and total simulation with climatic data for same seasons of previous years. The model output is then subjected to statistical analysis to guess the flood risk and possibility of reservoir refiling.
Extension of runoff records, Quality control and filling in of gaps
HBV model is used for the control of quality of runoff data. It also has proved to be useful for improvement of the effects of the ice-jamming on records. This model’s inputs helps to compare the change in snowmelt / rain and observed water level. In many cases, this model helps to identify the inhomogeneity in runoff records. The main application of hydrological models are filling in the gaps and extension of the runoff record. These methods are applicable in those areas where complete climatological records are available than hydrological records.
Design floods
Advanced guidelines for calculation of spillway design flood was adopted in 1990 in Sweden. They decided that hydrological modeling with reservoir simulation was most viable method for multiple reservoir system with rain flood and snow melt. These guidelines were based on regional design sequence of precipitation, time and elevation of year, basin size and hydrologic model. Most critical timing of all the floods producing factors are found by iterative simulation precipitation. The design of 10 years was inserted at all imaginable locations and response of reservoir level is analyzed.
Synoptic water balance mapping
SMHI have developed operational synoptic water balance map. This map demonstrates the hydrological condition in country by the symbols explaining soil moisture deficit, snowpack and runoff generation. It has showed to be as beneficial tool get rapid overview of hydrological condition in the country.
