| Soil erosion,as the greatest challenge of the sustainable development of the world,is even more serious in African regions where there are many developing and agricultural countries.Rwanda,located on the Plateau of East Africa,is a developing agricultural country.Agriculture is not only responsible for the country's food security issues,but also serves the country's economic development.However,under the pressure of global climate change,increasing population pressure,and rapid economic development,the soil erosion in Rwanda is becoming more and more serious.This has caused a series of problems such as grassland degradation and reduced farmland soil fertility,which in turn led to a decline in farmland productivity;At the same time,under the pressure of rapid population growth,disorderly cultivation began,and a large number of unreasonable cultivation further aggravated soil erosion.And this will create a situation of "the poorer the more cultivated,the more cultivated the poorer",which will have a huge negative impact on sustainable development.Aiming at the current situation of insufficient research on the law of soil erosion in the East African Plateau in Rwanda and its impact on sustainable development.This thesis selects the entire territory of Rwanda as the research area,and adopts a series of methods such as model simulation,trend analysis,grey correlation analysis,land use forecasting and scenario setting,etc.Based on the analysis of soil erosion intensity and land use changes at four time nodes in Rwanda in 1990,2000,2010 and 2015,Scenarios are used to simulate Rwanda's food self-sufficiency and soil erosion intensity under different development models in the next 30 years.The research results can provide a certain scientific reference or case demonstration for the sustainable development of Rwanda/East Africa under high population pressure.The main research conclusions are as follows:(1)Clarified the soil erosion intensity and its changing pattern at four time nodes in Rwanda in 1990,2000,2010 and 2015.The soil erosion modulus of Rwanda is generally increasing.The annual average soil erosion modulus in 1990,2000,2010 and 2015 were 25.49 t ha-1 y-1,34.23 t ha-1 y-1,34.07 t ha-1 y-1,and 35.51 t ha-1 y-1 respectively.However,the growth rate of the erosion modulus in the three stages gradually decreases,and the order is 0.87 t ha-1 y-1 y-1?-0.02 t ha-1 y-1 y-1,and 0.29 t ha-1 y-1 y-1.Soil erosion from croplands in Rwanda accounts for 62.98-80.78%of the total soil erosion and is the main source of soil erosion.In terms of space,the northern province has the highest soil erosion intensity,with an erosion modulus of 50.72 t ha-1 y-1.The Rulindo district in the Northern Province and the Bugesera district in the Eastern Province are the two districts with the largest and smallest soil erosion intensity in Rwanda,respectively.The soil erosion in Rwanda is mainly low(1-10t ha-1 y-1),followed by moderate,high,very high,severe and very low.With the passage of time,the soil erosion in Rwanda.The erosion level gradually shifts to a higher level.The soil erosion caused by the 1500-1900 m altitude zone accounts for the total amount of soil erosion 49.10%.The 15-25°slope area contributed 42.07%of the soil erosion in Rwanda.(2)Revealed the impact of climate change and human activities in Rwanda on soil erosion.The precipitation in Rwanda changed significantly from 1981 to 2015,and precipitation changes in about 80%of the region showed a significant trend of change(p<0.05).Rwanda is divided into humid and dry regions according to the positive or negative trend rate of precipitation,the proportions of the two areas are 47.45%and 52.55%,respectively.The dry regions are mainly distributed in the western and southern regions of Rwanda,while the humid regions are mostly distributed in the eastern,central and northern regions of Rwanda.The quantification of the intensity of human activity in Rwanda shows that the intensity of human activity in Rwanda is generally high,with an average human activity intensity of 23.87(the maximum is 35).The ratio of areas with high average human activity intensity to areas below average is 58.09:41.91.The intensity of human activities in spatial nature reserves(Nyungwe National Forest Park,Akagera National Park,etc.)is low,while the maximum human activity intensity in Kigali is 28.83.In general,the impact of climate change in Rwanda on soil erosion is greater than that of human activities.The gray correlation grade of climate change is 0.88,while that of human activities is 0.60.Spatially,nature reserves are the most sensitive areas affected by climate change,while the more sensitive areas affected by human activities are mainly the Nyabarongo Basin,the folds of the Northern Province,and the buffer zones on both sides of the rivers.(3)Expounded the land use change and its driving factors in Rwanda from 1990 to 2015,and predicted the land use pattern of Rwanda in the next 30 years.The land use types in Rwanda is mainly forest,grassland and cropland,which account for more than 89%of the total.Among them,forest was the main land use type in Rwanda in 1990,accounting for 42.95%.After 2000,cropland became the main land use type in Rwanda,accounting for 44.40%(2000)and 44.20%(2010)And 53.01%(2015).As the main land types in Rwanda,forest,grassland and cropland have been converted frequently.The land use change in Rwanda has undergone three main processes.One is the transfer of a large amount of foprest and grassland to cropland from 1990 to 2000,72%of the newly transferred cropland comes from the transfer of forest,and 28%comes from the transfer of grassland.The second is that between 2000 and 2010,the transition between the three basically maintained internal balance.The third is that after 2010,the area of cropland in Rwanda once again increased significantly and the grassland increased slightly,while the increase in cropland and grassland area was at the cost of the reduction of forest,of which 83%of the transferred forest became cropland,17%becomes grassland.The driving factors of land use in Rwanda have undergone significant changes in phase A(1990-2000),phase B(1990-2000)and phase C(2000-2015).Among them,changes in wetlands and waters are more affected by topography,changes in urbans are mainly affected by population,and changes in unused land are more driven by precipitation and soil factors.For forest,phases A and B are more driven by changes in precipitation,while phase C is the most important factor driving forest changes.The main driving force of grassland is the change from the population of phases A and B to the precipitation of phase C.The driving factor of cropland is population as the main in all stages,but in stage C,the topography limits and gradually weakens the change of cropland.The land use in Rwanda in the next 30 years will be mainly based on the continuous reduction of forest and the gradual increase in the area of cropland and grassland,followed by the gradual expansion of urbans.From a spatial perspective,in the future,the cropland in Rwanda will increase in the west and decrease in the east,while the change of grassland will be opposite to that of cropland.Therefore,the cropland will move westward and the grassland will move eastward.(4)Clarified the changes in crop production and population in Rwanda,and on this basis,Rwandan food crop production and soil erosion intensity are predicted for the next 30 years.Rwanda' s total crop production have shown a significant increase over time(p<0.05),and the crop production of the four types(food crops,vegetable crops,cash crops and fruit crops)have increased significantly(p<0.05).As the main crop type in Rwanda,food crops account for 87%of the total crop production in Rwanda.The structure of food crops has also changed over time.Before 2000,food bananas were mainly used as the main food crop.After 2000,with the planting of root and tuber crops,the root and tuber crops gradually replaced food bananas and became Rwandan main food crops.The structure of vegetable crops and fruit crops has gradually become more diversified as new species are suitable.As the structural change of the economy,sugarcane has replaced coffee as the main cash crop in Rwanda,while peanuts and tea have not changed much.The population of Rwanda is growing in general and conforms to the Logistic regression model.Its change process is generally divided into three stages.Before 1991,1991-2001 and after 2001,the population growth rate of the three stages was 3.1%,1.2%and 2.6%,respectively.The 2012 population census showed that Rwanda had a population of 1051.60×104 at that time,the ratio of rural population to urban population was 83:17,and the number of male was lower than the female,the ratio of the two was 91:100.The average age of the Rwandan is 22.7 years.In terms of spatial distribution,the population density of Rwanda in the secondary administrative region is between 178-2178 persons/km2.With the intensification of human-land conflicts in Rwanda,through the establishment of potential scenarios,baseline scenarios and conservational scenarios,the crop production and soil erosion intensity were simulated when Rwanda reached the middle of this century.The results show that through banana manure pits and mulching,trenches combined with living hedges or grass lines,and intercropping methods,the configuration of measures under the BS50XJ30 scenario under the baseline scenarios can enable Rwanda to control soil erosion intensity at 33.50 t ha-1 y-1 t while ensuring food self-sufficiency in the middle of this century.Under the conservational scenarios,the CS25XJ80 scenario(return cropland above 25°to forest or grassland)can enable Rwanda to reach the level of food self-sufficiency by 2050,and the soil can actually be controlled at 27.05 t ha-1y-1. |
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