| Three Gorges Reservoir Basin (TGRB) is a fragile Eco-economy area in China, where the agriculture is mainly dry farming cereals. The slope farmland is the main agriculture land use, and precipitation is the key resources for agriculture. The total precipitation is plenty; however, the uneven distribution in time and space makes the seasonal drought a serious problem in this region, which greatly hinders the sustainable development of agriculture. The slope land of purple soil, with frequent cultivation and human disturbance, is facing serious erosion. The rain-fed agriculture in this thin soil layer makes the water the key factor of productivity. Hence, how to manage the precipitation resources efficiently and match the crop water requirement with rainfall is a critical point in dealing with the seasonal drought and plant growth. Literatures shows that the farmland water balance is not only theoretically, but also practically to direct the real agricultural production. However, the research in China was mainly focused on arid and semi-arid areas like North China Plain and loess plateau, seldom seen in the southern part with seasonal drought, especially, in the TGRB’s purple soil area. Currently, the research in the slope farmland of purple soil is mainly on runoff, but less attention paid on evapotranspiration (ET), especially crop ET. In order to fill the research gap on farmland water balance in Southern part of China, this paper chose the ShiWan catchment’s slope land in KaiXian County of Chongqing as the study area, using the purple soil as the testing samples, to research on farmland water balance. This research will help the future work on purple soil water balance, its monitoring and forecasting, at the same time, hopefully it can help the categorization of slope farmland, provide the scientific background to integrating rainfall collecting, storage, supply and management in southern China part.Aiming to providing scientific basis for solving the seasonal drought problem, this study performed the pot experiment during2007～2012, pit-test experiment, and runoff plot experiment, integrating water balance, slope runoff generation theory, using field observation and theoretical analysis and calculation, the water balance and various components were analyzed in the purple soil farmland in TGRB. In order to reflect the real farmland situation, the regular wheat, oil-seed, maize and sweet potato and rotation combination of wheat/maize‖sweet potato, oil-seed/maize‖sweet potato were arranged during the experiment. The main results are as follows:(1) The infiltration process was dominated by the variation of rainfall intensity.During the growing season of2007to2012, it was shown that the summer and autumn had more rainfall, while less in winter and spring. Furthermore, there are usually rainstorms in summer and autumn and the rain were prone to cluster. The rainfall during June to August accounted for43.4%of the annual rainfall, and July and August rainfall accounted for55.2%and44.5%of the monthly average values. The statistical parameters Cv and Cs showed that the rainfall distribution were uneven during the growing season, one-sample K-S test showing that the monthly rainfall being with normal distribution.The single point infiltration varied in space and time in the slope land. The initial and stable infiltration rates had huge difference in different location of the slope land. For the60mins duration, the simulated rate in the end of1min ranges from8.03to20.83mm.min-1(Kostiakov simulation), and the stable rate ranges from0.76to3.81mm.min-1, an average value of1.42mm.min-1, and the cultivation had an important effect on infiltration. The infiltration rate decreased with time, during the period of0to10mins, the infiltration decreased fast with large amplitude; for the period10to25mins, the decreasing rate slowed down. After25mins, the rate starts to be stable and reached constant during27.7-35.4mins. The infiltration process can be simulated by Kostiakov formula, the infiltration rate a in the end of1min inversely correlated with initial soil water content, the infiltration rate and accumulated infiltration can be simulated by:The soil infiltration characteristics had a close relationship with rainfall intensity, the infiltration rate was dominated by rainfall intensity, and furthermore, the slope land infiltration was affected by many factors, which showed an unstable and non-continuous situation on both temporal and spatial scales. The rainfall cannot meet the infiltration during the beginning and end of the storm, which means f(t)=i(t) during this period. During the storm, when i(t)≥fp(t),f(t)=fp(t), but with short time interval and decrease with time. When i(t)≤fp(t), f(t)=i(t), all the rain infiltrated. With rainfall increase, the infiltration increases until to the maximum, and then decreases again. This process was dependent on the rainfall duration, infiltration characteristics and rainfall intensity.(2) Simulation with different time-interval can best reflect the runoff generation process during natural rainfall condition.The various factor like rainfall characteristics, antecedent soil water content and cultivation method had impact on the amount, time and component of runoff. With early peak and high intensity rainfall, runoff came early with large amount; if with high initial soil water content and long duration of low intensity rainfall after storm, the interflow accounted a significant amount. With the peak rainfall in the middle, high initial soil water content, even with normal intensity rain, if it lasts long enough, there would also be high runoff and interflow. However, with late peak rainstorm, if the previous rainfall being not replenishing the soil clearly, the rain will first satisfy the soil water moisture. Under this situation, only when the rainfall intensity exceeds the infiltration rate, there occurs runoff. If with low rainfall and short time, the loss will be huge and runoff and interflow are less.The surface runoff was mainly from infiltration excess runoff, and the runoff was during July and August. There are quite developed interflow in the study area, which can be found in May, June and September with a runoff coefficient of0.15-0.34. This interflow had no direct relation with rainfall intensity; however, it is related with antecedent soil moisture, saturation soil water and low intensity rainfall. The runoff mainly came during May to September and the monthly runoff coefficient is0.15,0.28,0.36,0.30and0.07separately. The relationship between rainfall and runoff can be expressed as: R=0.559P-44.03R2=0.948Based on the kinematic wave theory, proportionality and superposition theory, the time-interval runoff simulation can best reflect the runoff generation process. The method can clarify the initial and end time point, at the same time simplified the whole process. The results showed that there was perfect match between simulated values with measured data. The simulated runoff process matched well for the precipitation peak at the front, middle and rear of the rainfall event, the simulated runoff amount being34.4mm,39.7mm and10.2mm, which was in accordance with the observed data.(3) The evapotranspiration (ETc) was calculated by crop coefficient.The radiation was determined by Hargreaves formula. The standard Penman-Monteith formula is based on energy balance and water-vapor diffusion process, which not only accounts for physiologic characteristics, but also integrates aero-dynamics variations. The formula takes into consideration of various factors influencing ET, and has strong theoretical background. The reference evapotranspiration (ETo) have two parts:one is radiation part ET0(rad), another is aero-dynamic part ET0(aero).The (ET0) was mainly contributed by ETo(aero), accounting for70%, while with ET0(rad) being30%. During the period of2001～2012, the monthly and annual ET0(aero), ET0varied with a similar trend with temperature, while opposite with sunshine hours. Correlation and sensitivity analysis showed that:the climatic factors (temperature and sunshine hours) displayed significant correlation with ETo, same correlation being found between sunshine hour and maximum temperature; while the saturation vapor pressure deficit was the comprehensive reflection of crop reference evapotranspiration from temperature and relative humidity. The maximum temperature (Tmax) is the most sensitive factors for ET0while the sunshine hour the least sensitive. The results showed that the effect of temperature had far exceeded the effect of sunshine hour, which was only an indirect factor. Among the factors, the maximum temperature (Tmax), maximum relative humidity (RHmax), minimum relative humidity (RHmin) are three most sensitive factors for ET0. The significance analysis showed that:Angstrom, Hargreaves formula and Chongqing empirical formula had significant difference with the level of α=0.05in calculating daily ET0(rad) and annual ET0(rad), while monthly ET0(rad) had no significant difference. While the radiation (Rs) method to calculate the three methods daily ET0, monthly ET0and annual ETo had no significant different with α=0.05. According comprehensive analysis, the reference evapotranspiration was determined by Penman-Monteith method, the radiation by Hargreaves formula.The determination of crop coefficient was done by dual-crop coefficient. Single crop coefficient is simple which takes the average of the three growing period of a crop. However, the results from different hydrological years have clear difference due to the initial growing period coefficient greatly affected by rainfall (or irrigation) situation. Dual crop water coefficient can not only reflect the transpiration and soil water evaporation, but also the temporal variation of the basal crop coefficient (Kcb) and the soil evaporation coefficient (Ke). Furthermore, the daily variation of dual crop water coefficient will help the determination of various time intervals (e.g. day’s, decade’s or month’s) crop coefficient and the coefficient during rotation and co-existence for two or more crops. By adopting the dual crop coefficient, the crop coefficient of wheat, oil seed, maize and sweet potato was0.92,0.93,1.06, and0.98.The determination of intercropping patterns crop coefficient was done by weighted sum method. For intercropping plantation, it is more complex as there is main crop and associate crop, furthermore, the two or more crops are not planted simultaneously, which makes it difficult to determine the coefficient. Here, a weighted sum method was proposed to calculate the monthly crop coefficient of wheat/maize‖sweet potato, oil seed/maize‖sweet potato, and hence solve the problem of difficulty to determine the coefficient issues. More specifically, based on the range and space of crops as the weight, multiplied by the crop’s coefficient, the sum will be used as the total crop coefficient, that is:Where K’being the coefficient of intercropping crops; Kci being the ith crop’s coefficient;fi being the ith crop’s weight;li being ith crop’s range; d, L being space and range; i being the crop species (i=1、2......n).(4) The crop water consumption in the purple slope farmland limited by soil water stress.Precipitation (P) is the main water source and evapotranspiration (ETc) is the main water consumption item in the purple slope farmland. The runoff (R) and deep percolation (Fd’) was during May to September. According the theoretical analysis, the water balance in the purple soil farmland is: P-ETc-R-Fd’=ΔWsUnder the standard situation, the water budget was negative(ΔW<0), there was water shortage. the crop water requirement for wheat, oil seed, maize, and sweet potato, and the intercropping of wheat/maize‖sweet potato, oil seed/maize‖sweet potato was359.4mm,359.1mm,684.4mm,841.3mm,1178.3mm, and1158.7mm separately, with a shortage of136.2mm、135.9mm、298.7mm、336.4mm、451.9mm、432.3mm, for each of them. The water budget was negative (ΔW<0), there was water shortage for the whole growing period, the main water shortage period was during February to April and July to September, coinciding with the spring drought, summer drought, midsummer drought and autumn drought situation in the region.Under natural precipitation situation, the crop water consumption limited by soil water stress. Under water stress situation, during2008to2012, the crop water requirement for wheat, oil seed, maize, and sweet potato was255.6mm、256.0mm、553.3mm、621.8mm, separately, with a water stress coefficient of0.85,0.84,0.87,0.82each. The degree of water stress in different growing season were different, there was serious water stress during middle season and late season. During water stress, the soil water content increased with increasing ET0, which means that with high ET0, the critical soil water content is high during water stress and vice versa. For wheat and oil seed, when the soil water content reached60%of field capacity, water stress occurred. For maize and sweet potato, the critical soil water content was most clearly affected by meteorological factors and the range of soil moisture being large as61.1%(ET0=3.68mm.d-1)～76.8%(ET0=10.76mm.d-1) for maize and51.2%(ET0=.66mm.d-1)～80.3%(ET0=11.42mm.d-1) for sweet potato.In summary, based on the kinematic wave theory, proportionality and superposition theory, and by using the average infiltration rate for a rainfall interval instead of instant infiltrate rate, the time-interval runoff simulation can best reflect the runoff generation process. The theory is not only simplified the simulation process, but also being easily applicable for real world situation. After further research, the Penman-Monteith formula was suggested to calculate ETo and Hargreaves formula to calculate radiation, and the crop coefficient was calculated by dual crop coefficient method, and weighted sum coefficient solved the problem of dual or rotation mode of crops to fulfill the research gap in purple hilly area. The soil water deficit in the purple slope farmland was extremely serious, and the crop water consumption limited by soil water stress. The research results not only enrich the water consumption issue, but also laid a strong base on farmland water balance study. However, the infiltration characteristic under natural precipitation, slope land Manning coefficient, water balance in short time interval and water balance under water shortage should be further explored. |
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