Study On The Regime Of Grassland Fires Based On Geographical Information System And Remote Sensing

Grassland is considered as one of the most important ecosystem in the land surface. Grassland has important function on agricultural productions, environmental protection, and ecological security. As a universal natural phenomenon, fires always have good or bad impact on the grassland since it occurred. Wildfire has become an indispensable disturbance factor about the development and succession of grassland in natural ecosystems.In China, the area of grassland account for 41.7% of total land area. Approximately, fire was occurred in 10 million Ha area of grassland annually. These fires caused great economical loss and large harmful effects on grassland ecosystem. Grassland fire manager need to enhance the studies of fire disturbance and to know the rules of fire occurrence and fire spread, to grasp the pattern of spatial and time distribution. In this article, we have some conclusions by studies of fuel combustibility, fuel gross, fire behavior, fire climate, probabilities of fire occurrences and fire risk zones:1) The process of grass fuels combustion was divided into three stages: The first stage is no flame pyrolysis;The second stage is combustion with flame and the third stage is carbonizing combustion. The second stage is the most important stage. The peaks of Heat Release Rate, Mass Loss Rate,Smoke Production Rate and the other parameters of grass fuels combustion occurred in this stage. The integrated combustion index of 10 dominant species in song-nen plain was calculated and the relative value is ranked: Chloris virgata Swartz > Puccinellia chinampoensis > Pocockia ruthenica > Calamagrostis epigejos > Hemarthria japonica > Aneurolepidium chinense > Phragmites communis Trirn > Lathyrus quinquenervius > Suaeda corniculata > Lespedeza davurica.2) Grass fuel is one of biologic fuels. Pyrolysis characteristic of grass fuel is dominated by the main components, hemicelluloses, cellulose and lignin. The reaction order of process is 1. The process of pyrolysis was divided into three stage: before 120℃, the first stage is dehydration process and the ratio of mass loss is proportional to water content; During 120– 360℃, the second stage is pyrolysis of hemicelluloses and cellulose. In this process, the mass of volatile component yield and the ratio of mass loss are biggest. After 360℃, the third stage is pyrolysis of lignin and other residues. Char is the main production. An integrated burnout index is calculated to express the pyrolysis characteristic. The pyrolysis rate of Aneurolepidium chinense is lowest and the sustainability is best.3) The behavior of grassland fire is affected by topography, weather conditions, fuel conditions and the bed of fuels. The rate of grassland fire spread increases with increasing fuel mass and the volume density of fuel bed. When the density of fuel reached 350g/m2, the rate of fire spread begin to decrease. The threshold value of grassland fire spread is 35% of water content of grass fuels. In uphill, when the slope increases 1 degree, the speed of fire spread increases 1.07 times; in downhill, when the slope increases 1 degree, the speed increases 1.005 times. Most factors of weather have influence on grassland fire spread. But the effect of wind is more significant than the other factors. The relationship between wind and fire spread is positive. The lower limit of grass fuel supported fire spread is decreased to 60g/ m2 under the effect of wind.4) The difference of spectral features between residue grass and soil in short infrared is significant. An index of residue grass was created and this method would improve the present research situation of residue fuel mass. Integrating land surface temperature and water content of residue grass fuel, a method was created to assess the fuel moisture with reversing land surface temperature with thermal infrared image.5) In Inner Mongolia, index of fire climate is higher in April, May, June, September and October. The fire season is no less than 7 months and no more than 9 months. The spatial distribution of index of grassland fire climate is decline from west to east in the study region. During 1970-2008, the center of the spatial distribution changed continually. From 1970s to 1980s, the center moved from northeast to southwest. After 1980s, the center moved from southwest to northeast continuously.6) Supported by spatial analyst of GIS and Logistic regression, the predicted model of assessing the probability of human-caused grassland fire occurrence was created. The probabilities have positive correlation with temperature and have negative correlation with precipitation, relative humidity, the distance to dirt roads, paved road railroad, villages, human building and the elevation. In Hulunbeir, the probabilities of human-caused grassland fire occurrence are higher in southwest than in the other areas. The main reasons caused this distribution are higher densities of villages and dirt roads and more human activities in southwest.7) Integrating grass fuel, fire climate, accessibilities and slope, a general scheme of assessing grassland fire risk was proposed. With spatial cluster analysis, the grassland fire risk zones were divided and the zone map of Hulunbeir was designed. In this region, the area with higher fire risk was more than 65%, distributed in the middle and east.