Managing Water To Agriculture Through In-Situ Rainfall Use And Storage Works

Rainfed agriculture plays and will continue to play a dominant role in providing food and livelihoods in the world with an ever increasing population. How to improve in-situ rainwater use and hence reduce the agricultural risk is an important task nowadays. Rainwater harvesting has been practiced for many years in many areas of the world, especially, in arid and semi-arid regions like the Loess Plateau, North America, Middle-East, sub-Saharan Africa etc. However, there is little research done in humid area, even less done in the hilly area in Southwest China. Furthermore, the existing research is mainly focus on rural domestic use in north part of China, also some in agricultural but only use traditional practices like contour bunds, pitting cultivation etc. No research was done by using the soil water storage characteristics, combined by typical hilly area's hydrological process to mitigate the seasonal drought. In the hilly area of Chongqing, the annual rainfall is relatively rich. However, with its spatial and temporal variation, the regional and seasonal drought occurs frequently, which leads to serious agricultural losses every year. Hence, the core scientific aspect in this research is in-situ rainfall and runoff harvesting through analyzing the soil water characteristics, integrating the GIS and hydrological model, to supplement rainfed agriculture during dry spells in hilly area of Chongqing. The study has a profound scientific and practical meaning in mitigating the seasonal drought in this region.By using the monitored soil data and measured soil samples, the soil characteristics, especially those closely related with in-situ rainfall use like temporal variation of soil water content, infiltration and evaporation, were analyzed. Then by using GIS, SCS-CN based hydrological model, the rainfall runoff process was simulated to predict runoff potential for harvesting. Finally, under different land use pattern, the crop water requirement was calculated and size of storage ponds were calculated for supplementary irrigation based on water balance approach. The main research findings are as follows: 1) According to the recently 100 years rainfall data, taking the precipitation anomaly percentage as the criteira, the year 2006 was a dry year and 2007 a normal year. For the dry year, the variations of soil moisture in all three layers (0-10cm,10-20cm, 20-40cm) were medium (10%30%), and weak in the other two layers (CV<10%). Hence, in the humid area, the seasonal variation is large for dry year and relative small for wet year. For monthly variation, in the year 2006, the overall trend of soil moisture was declined, although there was a slight recovery from May to June. This was mainly due to the high temperature and evaporation, the actively crop growth worsen the situation. In the year 2007, the soil moisture fluctuates like a "W", from March to May, high crop water requirement made soil water decreased rapidly, from May to July, although crop water requirement was still high, with the increasing rainfall, the soil water content increased. From July, similar situation occur. In summary, monthly variation can be divided into high crop consumption and low rainfall period (March to May) and fluctuation period (June to September). For decadal variation, no matter for dry year or normal year, there was a drastic fluctuation of the soil water content. However, due to the impact of 2006 drought, the fluctuation of soil moisture in 2006 was much bigger than that in 2007. For five-day variation, the upper layer (0-10cm) was the most sensitive layer, and there were clear difference between different layers.For the probability distribution of soil moisture, no matter for the dry year or normal year, all the distribution in three layers have a single peak shape. However, the location of the peak appeared at different position in different layers and years, similar with the band of the peak.2) Among the various factors, the presence of rock fragments modifies the soil physical properties such as available water content, infiltration and runoff susceptibility. The results of laboratory experiments showed the effects of rock fragments contained in three different purple soils of the Sichuan basin of southwest China. The experiments investigated how these rock fragments alter the soil's physical, chemical and agronomical characteristics such as infiltration and evaporation. We found that the infiltration rate, whether horizontal or vertical, in the three soils has the following order: grey brown purple soil>reddish brown purple soil>brown purple soil. With increasing rock fragment contents the accumulated infiltration decreases, while the total time decreases first and then increases. The minimum occurs at approximately 10% to 20% of fragment content by weight. The infiltration rate also changes with the distance. In the 0-5cm range, the initial infiltration rate increases with increasing rock fragment contents, in the 5～10cm range, the slope of infiltration curve increases with increasing rock fragment contents. With increasing distance, the slope gradually decreases and finally reaches a stable value. The presence of rock fragments reduces soil water content, with the minimal value appearing when the rock fragments where on top of the soil column, while decreasing with increasing rock fragments with soil samples mixed with fragments. Under the constant 40℃temperature, the accumulated evaporation and evaporation rate are minimal for soils covered by rock fragments, and the accumulated evaporation decreases with increasing rock fragment for other soil samples. However, the evaporation rate increases with increasing rock fragments in the first 4 days and decreases thereafter.3) The GIS coupled with SCS-CN model was applied in this study to calculate runoff in each micro-catchment. The monthly average rainfall from 2000～2005 were analyzed and used to calculate the runoff. The CN number of each catchment is calculated based on its soil groups, land use areas using weighted mean method. The results showed that with different CN number, the runoff is different even with the same soil conditions. The runoff concentrated in the period of April to October which stands more than 90 percent of total annual runoff. However, the runoff from July and August only take a small part of total yearly runoff, which is closely related with the average rainfall of these two months; possibly there is certain relation with evaporation, transpiration, and infiltration. The coefficients of uneven distribution for annual runoff in the year of 2000 to 2005 were 0.97,1.25,1.15,1.10,1.24 and 0.97 respectively, however, the coefficient got from 6 years monthly average data is 0.90, which is much smaller than any of the value in the years. The results illustrates that the variation of runoff is different in the years, with 2000 and 2005 being relative small, and 2001 and 2004 rather big. The average runoff coefficient during this 6 years'period is 0.49, with the coefficient of variation of 0.27. The high runoff coefficient is consistent with the fact of high runoff in southern part of China. However, compared with the 0.51 runoff coefficient generally used in Chongqing area, there is in fact some difference. For example the smallest value is 0.4 in 2001, and a large value of 0.54 in 2004. Hence, in the study for more accurate runoff calculation, the rainfall runoff process should be further studied.It is a key issue for regulating and harvesting local runoff in hilly area. The slope to terrace project can increase the residence time of rain water on the soil surface and hence increase the infiltration time. In this study, three categories of slope had been lowered. The soil layer had been increased 20-30cm, and the residence time increased 2 to 60 seconds. The soil water storage increased 168 to 245 m3/hm2 for different slope change, which greatly helps the crop to abstract water from soils.In addition, according to the existing reservoir and water storage facilities, the slope to terrace project to increase soil water storage, and runoff potential analysis, crop water requirement, the water balance was made to design the size of water storage cisterns to supplement irrigation in each sub-catchment. The size of the cisterns is usually 100m3, with some small ones in catchments which need less water for crop requirements.4) In order to mitigate the seasonal drought, in this study we analyzed the drought characteristics from both meteorological and soil aspects. The probabilities of drought with different severities were calculated and compared with the stationary distribution probabilities. The results showed that both the spring drought and dog-day drought had the probability greater than 60%. For crop water requirements under different land use pattern and slope, we found that there was close relationship between these variables. As soil is a natural reservoir to store water for plant use, we also investigated the soil water characteristics, different soil reservoir capacities and their variation at different locations on the slope. After the analysis of the supply side (rainfall, drought characteristics and soil reservoir) and demand side (crop evapotranspiration) were performed, a balance was tried between supply and demand. We found that in the dryland agriculture, 27 storage cisterns was needed to mitigate seasonal drought in the study area; among which 8 should have a capacity of 100m3 and 19 with a capacity of 50m3. For paddy field, we need to establish bunds to form a totally 5.05hm2 fields to store 1.57*104m3 water. However, the fields should be sparsely distributed in order to easily supply water to different farms. Implications are that detailed meteorological and hydrological data are needed to make the results sound. However, for a local area, these data are usually difficult to obtain. Assumptions were made for some special situations in this study.In summary, in-situ rainfall use is an efficient way to mitigate seasonal drought and hence improve reliability of water to agriculture. Rainfall and runoff harvesting is commonly used in arid and semi-arid regions, however, interest in rainwater harvesting increases in humid regions, even areas containing well developed water supply infrastructures. In the hilly areas of Chongqing, seasonal droughts occur frequently and the water supply infrastructure is quite poor. Therefore, a better understanding of soil water dynamics, controlling factors influencing soil water recharge and discharge is critical for better using rainwater in-situ. Rainfall runoff process can help identify the potential of harvesting. Slope to terrace project can improve soil water storage and hence increase rainwater use efficiency. The remaining rainwater after infiltration and evaporation can be stored in small-scale reservoirs or cisterns for supplementary irrigation during seasonal drought, which reduces the risks of rainfed agriculture apparently in this region.