مقايسه روش‌هاي تعيين دماي آستانه و مناطق پتانسيل ريزش برف در حوضه‌هاي آبريز كرخه، دز، كارون و مارون

Comparison of methods for determining the snowfall threshold temperature and potential area affected by snowfall in the Karkheh, Dez, Karoon and Maroon river basins

دماي آستانه ريزش برف از جمله عوامل موثر در تعيين نوع و ميزان بارش در حوضه‌هاي كوهستاني محسوب مي‌شود. در اين مقاله، با استفاده از داده‌هاي متوسط روزانه دما و مشاهدات نوع بارش و مقدار برف و باران، دماي آستانه و سطوح پتانسيل ريزش برف در 18 ايستگاه سينوپتيك حوضه‌هاي كرخه، دز، كارون و مارون تعيين و الگوي تغييرات آن به‌صورت مكاني و زماني مشخص شد. نتايج نشان داد كه ميانگين دماي آستانه ريزش برف براي كل منطقه و كليه ماه‌ها حدود ?C4/2 است. همچنين بررسي ماهانه دماي آستانه ريزش برف نشان از وقوع بيشترين مقدار دماي آستانه (?C8/2) در بهمن ماه است. با توجه به آمار ايستگاه‌هاي برف‌سنجي منطقه، بهمن ماه برف‌گيرترين ماه سال است. بررسي روابط دماي آستانه بيانگر عدم ارتباط اين كميت با ارتفاع بوده اما تا حدودي با طول و عرض جغرافيايي وابستگي نشان مي‌دهد. با استفاده از كميت‌هاي دماي آستانه ريزش و ذوب برف و نقشه‌هاي هم دماي ماهانه، ‌سطوح پتانسيل تحت ريزش، ذوب و استقرار برف به تفكيك ماه‌هاي مختلف براي حوضه‌ها تهيه شد. سپس روند تغييرات مكاني مناطق زير پوشش و ذوب برف ماهانه در گام‌هاي ارتفاعي 250 متري براي هر حوضه در سامانه اطلاعات جغرافيايي GIS تعيين شد.

Snowfall Threshold Temperature (STT) is used for determining the form of precipitation as snow or rain and is applied for estimating the amounts of stored water resources in mountainous watershed. It seems to know this parameter is essential to secure water resources management. The SST plays a key role in snowmelt model and hydrologic simulation. A wide range of studies in different areas, show that this parameter varies between 0 to 5°C (Masujiro and Nakamura, 1997; McCabe and Wolock 1999; Tolson and Shoemaker, 2004; Lijuan et al. 2013 and etc.). Masujiro and Nakamura, (1997) reported that any precipitation above 2.7°C in mountainous area of Japan falls in the form of rainfall and below 0.7°C falls in the form of snow. Tolson and Shoemaker (2004) showed that this range is between 0 to 2.2°C in the Cannonsville river basin of the USA. Lijuan et al. (2013) evaluate the effects of SST on simulated snow depth by two different models of CLM and CLM-WRF. This study demonstrated that both models have good simulation result using SST of 2.5°C. In this study, using daily average temperature, amount and type of precipitation in the daily scale, the STT has been estimated for Karkheh, Dez, Karoun and Maroun river basins and the temporal and spatial variation of STT is addressed. The study area is located in Zagros Mountains that is one of the main snowy region in Iran and provide major water of several large rivers of Iran such as Karkheh, Dez, Karoon and Maroon and supply water for more than 7 million populations. The daily, hourly and monthly meteorological data from 18 synoptic stations scattered in different locations and elevations are analyzed. For each synoptic station, type of precipitation, temperature and depth of snow were available every 3 hours (00, 03, 06, 09, 12, 15, 18 and 21), while the amount of precipitation was available for every 6 hours (00, 06, 12 and 18). Therefore, the accuracy of estimated snow water equivalent (SWE) is less than 6 hours. Using the mentioned data, daily snow water equivalent is estimated and based on this parameter, the ratio of SWE to total precipitation is calculated. This parameter is plotted for different month and different station. The SST was determined using the mean daily temperature and hourly amount and type of precipitation (solid or liquid) at the Kermanshah synoptic station. This parameter considering mean daily temperature, a small amount of snowfall may occur at temperatures above 5 °C. Conversely, a small amount of rainfall below 0°C has been observed. The temperature of 50% snowfall was chosen as the initial value of the SST. Results showed that the overall STT for the entire study area and total months is about 2.4 °C. The maximum STT belongs to the month of February with 2.8 °C, also based on the data analysis, this month has the highest recorded amount of snowfall. The linear correlation between STT and altitude, longitude and latitude shows that the STT has no relationship with elevation, but it seems to be related to longitude and latitude. Using the Snowmelt Threshold Temperature (STT) and monthly isothermal maps, the monthly maps of the potential snow pack area, snowmelt area and snowfall area is prepared. The results showed that in Februarys most of the study area has the potential of snowfall. However, most of this snow probably is melted, immediately. The spatial variation of the potential snow pack and snow melt area has been addressed with 250 meter elevation boundary using GIS tools. In this study, the sensitivity of the snow cover area to SST is evaluated. Results revealed that 1°C decrease in SST may lead to 13 % decrease in snow cover of entire study areas. Also, the 1°C increase in SST may lead to 17 % increase in the snow cover area of entire study areas. Remarkably, the Karkheh river basin has the most sensitivity to SST. One centigrade degree increase or decrease in SST may lead to 34 % snow cover increase or 26 % snow cover decrease for this river basin, respectively.