Water Resources Systems—Hydrological Risk, Management and Development (Proceedings of symposium 90 IIS02b held during IUGG2003 at Sapporo, July 2003). IAHS Publ. no. 281, 2003. Flood forecasting and management in Pakistan SHAUKAT ALI AWAN Flood Forecasting Division, 46 Jail Road, Lahore 54000, Pakistan [email protected] Abstract Meteorologically, there are two situations which may cause three types of floods in Pakistan: (i) meteorological situation for Category-I floods, the seasonal low is a semi-permanent weather system situated over south­ eastern Baluchistan, southwestern Punjab, adjoining parts of Sindh, which intensifies and causes moisture from the Arabian Sea to be brought up to the upper catchments of the Chenab and Jhelum rivers; (ii) the situation for the more severe Category-II and Category-Ill floods, is linked with the monsoon low/depression. Such monsoon systems originate in the Bay of Bengal region and then move across India in a general west/northwesterly direction to arrive over Rajasthan or the adjoining states of India. Flood management in Pakistan is a multi-functional process involving a number of different organizations. The first step in the process is issuance of a flood forecast/warning which is prepared by the Pakistan Meteorological Department (PMD) utilizing satellite cloud pictures and quantitative precipitation measurements using radar data, in addition to the conventional weather forecasting facilities. For quantitative flood forecasting, hydrological data is obtained through the Provincial Irrigation Department and WAPDA. Furthermore, improved rainfall-runoff and flood routing models have been developed to provide more reliable and explicit flood information to a flood prone population. Key words Category I, II and III floods; monsoon low/depression; Pakistan INTRODUCTION Floods in Pakistan are mainly caused by the heavy monsoon rains during the summer monsoon period, from July to September (Thompson & Perry, 1997). Meteorologically there are two situations which cause flood producing rains in the upper catchments of the rivers (National Engineering Services, 1998). These two situations are related to different conditions of intensity and movement of lows/depressions, and produce three categories of floods: (i) Meteorological situation for Category-I floods: the seasonal low is a semi-permanent weather system situated over southeastern Baluchistan, south­ western Punjab, adjoining parts of Sindh, which gets intensified (due to a westerly wave) causing moisture from the Arabian Sea to be brought up to the upper catchments of the Chenab and Jhelum rivers resulting in heavy rainfalls along windward slopes of the mountain ranges due to orographic uplift of the moist air masses, (ii) Situation for Category-II and Category-III floods: this generating situation is linked with monsoon lows/depressions. Such monsoon systems originate in the Bay of Bengal region and then move across India in a west/northwesterly direction and arrive over Rajasthan or any of the neighbouring states of India. After the monsoon, the depression may take any of following three courses: (a) continue moving straight west causing heavy widespread rains over Sindh/Baluchistan, in which case no river flooding will occur; (b) turn to the northeast towards the upper catchments of the Sutlej, Ravi and Chenab Flood forecasting and management in Pakistan 91 rivers causing extremely heavy rainfall and consequently floods, first across the border in India and then (within hours) at the Rim Stations in Pakistan. This is a Category-II flood situation; (c) continue moving in a northerly direction under the effect of strong westerly winds over the plains of the Lahore/Gujranwala Divisions, to finally end up over the Rawalpindi/Hazara Divisions. The upper catchments of the Chenab, Jhelum and Indus rivers then come under its influence. The flood forecasting system in Pakistan comprises three components: (i) a meteorological component which relates to the forecasting of precipitation; (ii) a hydrometeorological component which relates to the generation of a flood wave at the Rim Stations; and (iii) a hydrological component which relates to routing the flood wave and forecasting flood peaks at downstream sites. METEOROLOGICAL COMPONENT The meteorological component is an extremely important part of any flood forecasting system, especially in tropical countries where floods are caused by rainfall only (with little or no snowmelt contribution). Flood producing rain is caused by the condensation of the moisture-laden monsoon air mass due to convection in a low-pressure area or due to orographic lifting across a mountain barrier, or both. The resulting super saturation of the cloud mass triggers precipitation, which depends upon the continued availability of a moist airflow into the precipitation region. In Pakistan, minor floods are caused by the penetration of the southeast and southwest monsoon currents, along the region of the active monsoon which includes the upper catchment of the Sutlej, Ravi, Chenab and Jhelum rivers. Major floods are, however, caused by the monsoon lows/depressions which have a long track stretching from the Bay of Bengal to the catchment areas of the rivers. The above two meteorological situations need to be closely monitored in order to develop the capability of predicting the timing and amount of the flood producing rains. The following meteorological methods are used for this purpose. Meteorological charts/maps As members of the World Meteorological Organization (WMO) most countries of the world are committed to observe, collect and transmit meteorological data from their respective countries in accordance with the schedule laid down by WMO. The schedule provides that, commencing from 00:00 h GMT, the meteorological data at all surface observatories is observed every three hours, while that of the upper air observatories is recorded at a six-hourly interval. This makes it possible for Pakistan to obtain the much needed meteorological data which includes such elements as pressure, temperature, rainfall, clouds, surface and upper winds, etc. This data, along with that from Pakistan and the other surrounding countries is then plotted on the meteorological surface and upper air charts and analysed to identify the lows/depressions, etc., or the extent, and intensity of the monsoon incursion into the catchment areas. This, even though an age-old conventional process, is still the most important means of meteorological forecasting. 92 Shaukat Ali Awan The depth of the low pressure for example, is an important indicator of the intensity and duration of the associated rainfall. In addition to the locally prepared meteorological maps/charts, those prepared at Tashkent (one of the WMO centres), and that prepared at Reading, UK (by the European Centre for Medium Range Forecasting) are also received as and when needed. A number of meteorological studies aimed at improving the methods of predicting rainfall from monsoon lows, and predicting the conditions of recurrence of monsoon lows in the catchment areas are in progress, while some are planned for the future. Weather satellite images There are two types of meteorological satellites in use for receiving the cloud pictures: the geostationary and the polar orbiting satellites (Donald, 2000). The geostationary satellites are more useful since they provide pictures at half hourly intervals (as they remain static relative to the earth and are thus continuously monitoring a very large region of the Earth under them). Perhaps Pakistan is the only country in the world today having no access to a geostationary satellites and thus has to rely on the use of polar orbiting satellites only. The orbits of polar orbiting satellites are continuously shifting with respect to the Earth. The amount of shift depends upon the time taken by the satellite in circling the Earth which is related to the orbital height. Presently, the two satellites in use are the NOAA and NOAA 14 satellites, which are orbiting the earth at a height of approx­ imately 850 km and thus take about 100 minutes to circle round the earth. This means that the Earth shifts by about 25.4 degrees eastward at about 25.4 degrees longitude, between each orbit, so progressively bringing new regions of the Earth under it. Polar orbiting satellites therefore have the advantage that a single satellite is able to cover all points on Earth. On the other hand, however, it has the disadvantage that long intervals of time occur between two passes over a particular location, providing only two to four usable passes in 24 hours. The two polar orbiting satellites together provide only two to six usable passes every 24 hours. Fortunately, the long awaited Russian satellite is about to be launched above the Indian Ocean at longitude 76°. This will be a geostationary satellite and will provide cloud pictures at half hourly intervals. The visible and the infrared (thermal) bands will be received to identify weather systems such as monsoon lows/depressions and the areas of extensive convective build-up within the monsoon air mass. It will be an excellent device for forecasting torrential rainfall and associated floods, particularly in connection with radar information. Use of weather radars in flood forecasting Radar is a remote sensing device capable of monitoring falling precipitation. It has a meteorological range up to which it can detect the existence of precipitation. Radar has a very special use in the context of flood forecasting in Pakistan in view of the fact that the upper catchments of most rivers lie across the border and are thus physically inaccessible for the installation of ground-based raingauges. Access to real-time Flood forecasting and management in Pakistan 93 ÂDAR Ir.liiiimlfirl ! Y.Kh.m I H r.H.n.hi m * " ": .Hahore- ÉêT. ..... t . '". " : Fig. 1 Areal coverage of weather radar network in Pakistan. rainfall data from the upper catchments in India is also not possible. The only alternative in this situation is to remotely sense the falling precipitation using quantitative precipitation radar. Consequently, a C-band 5.32-cm wavelength QPM radar was installed at Sialkot in 1977.