Experimental Simulation Technique of Rainwater Harvesting Modes. Optimization on Small Watershed of Loess Plateau in China

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1 Experimental Simulation Technique of Rainwater Harvesting Modes Optimization on Small Watershed of Loess Plateau in China J.E.Gao*, S.W.Yang*, H.Shao*, Y.X. Zhang,* M.J. Ji * * Northwest A&F University, Institute of Soil and Water Conservation, C A S& MWR, National Engineering Research Centre for Water Saving Irrigation at Yangling, Xinong road26,yangling, 72, Shaanxi,China( gaojianen@26.com) ABSTRACT Aiming at how to optimize rainwater use on small watershed of the loess plateau in China by experiment simulation, a design means and experimentation technology and its verification test were given based on similarity theory and hydrodynamic principles of rainfall, runoff and infiltration. The model is one of Kangjiagou small watershed of the loess plateau of China. It complied with the similarities of geometric, rainfall, flow, erosion production sediment transport and bed deformation etc. The verifying experiment results indicated that the movements of rainfall, flow, and production sediment and bed deformation agreed to the practical situation. It is concluded that the bad water erosion appeared in July to September and were caused by four to six heavy rains. Centralizing rainfall is an advantage for rainwater harvesting to prevent drought and water erosion. The similarity experiment is an efficient tool for laboratory forecast. It can be used to optimize rainwater utilization mode and control soil erosion and utilize water and soil resource effectively. KEYWORDS Rainwater harvesting; small watershed; simulative experiment; design and verification INTRODUCTION Surface flow control through varied types of rainwater harvesting is the key to resolve the contradiction between drought, and soil and water loss, in the Loess Plateau. The essence of the indoor model investigation about hydraulic erosion movement in small watershed is to forecast the actual sediment source of rainfall and runoff erosion, optimize utilization measures, control soil and water loss, and achieve the goal of utilizing soil and water resource efficiently. In order to make quantitative forecast, the model must be similar to the prototype. It demands that model must have a correct design theory and further could be verified by the prototype information besides satisfying the requirement of correct similarity scale (Gao, 25, 26). However, because the simulative experiment on hydraulic erosion in the field of agricultural engineering was limited by the condition of similarity and design theories, the verification of similarity was always a difficult point in domestic and overseas studies (Mamisao, 952; Chery, 965; Grace and Eagleson, 965; Zhou Wende, 969; Zhu Xian, 957; Jiang et al., 994; Yuan et

2 al., 2; Shi et al., 997). Based on new views about rainfall and runoff control through rainwater use and simulative experiment theories (Gao, 25,26), Kangjiagelao, a small watershed in the Loess Plateau, was chosen as a prototype to design and verify the simulative theories. The simulated small watershed covers an area of.347 km 2. Its gully density is 3-4 per km 2. The length of the watershed is.93 km, and the widest part of the watershed is.723 km, with an average width of.52 km. The watershed shape coefficient is.52 and the maximum relative elevation is 9 m. The soil erosion modulus is 9, t/ (km 2.a), so it is a region of intense soil and water loss. The soil erosion in the small watershed has developed into its own system. Its rainwater utilization types are varied and the ecosystem is easy to be controlled. Moreover, there are sufficient observation data for comparison study. MODEL DESIGN About the model design, we first obtained the basic scale of rainfall and runoff similarity (see Table ) under the condition of similarity of geometric, rainfall, flow, sediment movement, and bed deformation being met (Gao, 25, 26). Here the similarities of river bed erosion and sediment movement for choosing model soil were referred mainly. In the mobile-bed model experiment, the condition of soil suspension similarity was mainly used to control the selection of model bed sand. The silt in the watershed had a median diameter of approximately.25 mm, and 98% of the silt less than. mm. Therefore, Stork s hydrostatic settlement formula could be used in the laminar regime. In considerations of suspension similarity and the ratio of inertial force and gravity, the grain scale is expressed by d = () / 4 l ν ρs ρ ρ where d is the grain scale, ν is kinematic viscosity coefficient scale, ρ ρ s ρ is the scale of silt submerged weight. For natural soil, if the geometric scale is, d = 3.6, d 5m =.25/3.6.8 mm. Equation () provided a reference to select the soil for the model. Figure showed the calculated and actual adopted model soil graduation when the model soil was natural soil and the geometric scale was. The starting similarity requires u = u (2) where u and u is the scale of the competent velocity and flow velocity. According to Hazen s data on overland flow (Gao,25; Liu and Wu 997), Fig. 2 gave the relationship between competent velocity and grain size. From the figure, it is clear that when d 5 =.25 mm in the prototype, the competent velocity (v y ) is.8 cm/s; but when d 5 =.759 mm in the model, the 2

3 u competent velocity (v m ) is.83 cm/s. So, =.8/.83, which satisfies the similarity requirement. Table. Main model scales Scale Name Scale Notation Scale Value Geometric similarity Plane scale Vertical scale Rainfall intensity scale l h = = i v l Rainfall similarity Rainfall amount scale = P i t 33.3 Flow movement similarity Rainfall time scale Velocity scale Water flow amount scale Coefficient of roughness scale Flow time scale ' t 3.3 v = l 5 / 2 Q = l / 6 n = l.47 t = l Suspension movement similarity d = / 4 l ν ρs ρ ρ 3.6 Sediment movement similarity Soil water similarity Competent velocity scale Sediment content scale Deformation time scale Sediment transporting ratio scale Soil water content scale Infiltration rate scale = = v o v l s 3 ' t 3.3 G 3 θ f = l 3

4 P(%) Antitype Diameter 4 3 Model Demand 2 Real Select... D(mm) Figure Soil Grain Grading Curves V (cm/s) D ( m Figure2 Relaton curve of Hazen starting velocity and grain size EQUIVALENT RAINFALL PROCESS AND CHOICE OF SEDIMENT CONTENT SCALE Equivalent rainfall process Actually, rainfall could be divided into valid rainfall and invalid rainfall. For the rainfall and runoff control, the purpose was reducing water erosion so as to utilize soil and water resources efficiently. Not all rainfall on the loess plateau could cause erosion, so the rainfall dispersing and transferring silt was defined as eroding rainfall, or valid rainfall. If rainfall with an erosion rate of. t/ km 2 per year was adopted as the criterion for eroding rainfall, it was determined that eroding rainfall per year in the north and middle area of Loess Plateau was 4 5 mm, and erosion was normally occurred 5-7 times a year. According to recent observations, this conclusion conformed to the real situation in the Kangjiagelao watershed. For instance, average rainfall amount in the watershed in 998 and 999 were, respectively, 57.4 mm and mm, and average number of eroding rainfall events were, respectively, 7 times and 6 times. Thus, the average eroding rainfall amount was 4 mm. Equivalent rainfall process means a continuous rainfall process with precipitation, duration and eroding amount equivalent to the corresponding value of prototype when rainfall intensity was given. It includes equivalent rainfall intensity, equivalent eroding amount, and equivalent rainfall duration. According to a comprehensive analysis, it was adopted that Wy = 8t - 9t was an 4

5 annual prototype erosion amount, and Iy =.4 mm/min (see Fig. 3) was an average eroding rainfall intensity of 7% guaranteed rate during minutes, and Ty = Py/Iy = 5/ min was an average eroding rainfall time. Thus, the model rainfall intensity of Im =.4 mm/min and average eroding time of Tm = Ty/t =3.6/3.3 4 min. Rainfall Intensity (mm/min) , years, years 5 years years 5 years 2 years years 5 years 2 years.4 years year. Ti me (min) Figure 3. Duration curves of rainfall intensity and exceedance interval Preliminary experiments and sediment content scale Based on the experiment design mentioned above, the model of the Kangjiagelao small watershed was built. Following a preparatory rainfall experiment, the sediment content scale was determined to be equal to 3. Thus, the similarity scale of the small watershed model can be determined (see Table ). PRELIMINARY VERIFICATION The model should be verified in rainfall, runoff, and erosion production sediment transport and bed deformation and so on, for forecasting antitype. They were done as following. () Rainfall similarity consists of rainfall intensity similarity and rainfall spectrum similarity. The verification test proved rainfall similarity was satisfied by controlling mainly the rainfall intensity similarity. However, it was easy (Gao, 25). (2) Under the condition of satisfying soil mechanical composition and rainfall similarity, an experiment was carried out in terms of an equivalent rainfall process with.4 mm/min of rainfall intensity, and 4-min rainfall duration. Figure 4 shows the experimental process which was switched into a prototype. Additionally, the conflux time, average conflux velocity, maximum flow amount and annual erosion amount were verified (see Table 2). Table 2 showed that, in the model of given scale, the conflux time, average conflux velocity, maximum flow amount, and 5

6 annual erosion amounts were close to that of the prototype. That is, the model could reflect the reality of hydraulic erosion in the prototype. Q(m 3 /s) Dischage Sediment 2 3 T(h) Fi 4 h h f Di h Figure 4 The Graphs of Discharge And Sediment Concentration S(kg/ m 3 ) P ( %) Model 5 Field 6 4 Field 5 3 Field 4 2 Field D (mm) Figure 5 Grading Verification (3)Sediment graduation is the comprehensive reflection on runoff, sediment production and transportation and riverbed deformation. Figure 5 shows the graduation comparisons of different spots sediment in Kangjiagelao watershed after a rainstorm (July 3, 23) with eroding sediment in model. Because the values of sediment graduation have no obvious difference, the result was still satisfactory, both qualitatively and quantitatively. Table 2. Equivalent rainfall intensity processes of the model and prototype Name Model Prototype Eroding rainfall amount (mm) Conflux time (h).3.2 Average conflux velocity (m/s) Maximum flow amount (m3/s) Annual erosion amount (t/km2) 2,9 2,8-3, OPTIMIZATION EXPERIMENT ON RAINWATER USE TYPES Based on the preliminary verification, some primary experiments were carried out to the existing runoff catchment engineering measures, the water cellar and terrace as Figure 6 and Table 3. The condition of experiment remained the same as the verification test in order to make a comparison. The optimizing experiment involving two rainwater use measures showed that terracing reduces runoff to 4% and sediment to % of the cellars collecting measures, whereas the area of the former was 2.7 times of the latter. In terms of the controlling ratio of unit area, it was obvious that the controlling ratio of the rainfall collecting measures was relatively high. Therefore, the small 6

7 watershed simulative experimental technology could be carried out to optimize scheme and determine different measures and their best combination way. Figure 6 The topographical map of Kajiagelao Small Watershed (measured in 23) Table 3. Equivalent rainfall intensity processes of the model and prototype Rainwater catchment type Area(km 2 ) Rate of the total area of the watershed (%) Cellar.6 8% Terraced field.62 47% CONCLUSIONS Based on the above analysis, the following conclusions can be made: () Not all the rainfall can cause erosion on the Loess Plateau, so the equivalent rainfall and runoff process can adopt in the hydraulic erosion experiment of small watershed. (2) The rainfall equivalent process consisted of equivalent rainfall intensity, equivalent erosion amount and equivalent rainfall time. Equivalent rainfall intensity can be adopted the maximum rainfall intensity of 3 minutes in a certain guaranteed rate as the experimental rainfall intensity. (3) The verifying experiment results show that when the model geometric scale was, the movement of rainfall, flow, sediment production, and transport was consistent of the real Kangjiagelao small watershed in Yan an. (4) The experiment show that rates of the cellar collecting rainwater and reducing sediment were higher than terrace. It indicates also that the technology can be used to optimize controlling measures, prevent soil erosion and utilize water and soil resources efficiently. 7

8 ACKNOWLEDGEMENT The research was financially supported by the Chinese th Five-Year National Key Technology R&D Program Plan of rainfall and runoff controlling and efficient utilizing technology on sloping surface (26BAD9B) and engineering mode of rainwater safe harvesting for single family in rural area of north plain (26BADB4-2). REFERENCES Gao J.E., Yang S.W., Wu P.T., Wang G.Z. and Shu R.J. (26). Preliminary Study on Similitude Law in Simulation Experiment Controlling Hydraulic Erosion [J]. Transactions of the Chinese Society of Agricultural Engineering. Vol.22, No., pp4~45. Gao J.E., Wu P.T., Niu W.Q., Feng H., Fan H.H. and Yang S.W. (25). Simulation Experiment Design and Verification of Controlling Water Erosion on Small Watershed of Loess Plateau [J]. Transactions of the Chinese Society of Agricultural Engineering. Vol.2, No., pp4~45. Mamisao J.P. (952). Development of Agricultural Watershed by Similitude[R]. M. Sc. Thesis, Iowa State College, pp. -3. Chery, D.L. (965). Construction, Instrumentation, and Preliminary Verification of a Physical Hydrological Model[R]. USDA- ARS and Utah State Univ Water Research Lab. Report. Logan, Utah, USA, pp. 5-. Grace R.A. and Eaglson P.S. (965). Similarity Criteria in the Surface Runoff Process[R]. MIT, Hydrodynamic Lab, Technical Report No. 77, pp Yen B.C. and Chow V.T. (969). A Laboratory Study of Surface Runoff due to Moving Rainstorms [J]. Water Resources Research, Vol.5, No.5, pp Zhu X. and Wen Z.R. (957). Verify Basic Assumption about Unit Line According to Indoor Catchments Model. Journal of Hydraulic Engineering, No. 2, pp. 7-. Chen Y.Z., Jing K. and Cai Q.G. (988). Modern Erosion and Harness in Loess Plateau [M], Science Publishing Company, Beijing, China. pp Jiang D.S., Zhou Q. and Fan X.K. (994). Normal Simulative Experiment on Water and Sediment Control in Small Watershed[J]. Journal of Soil and Water Conservation, No.6, pp Yuan J.P., Lei T.W.and Jiang D.S. (2). Small Watershed Model Experiment in Different Harness Degree [J]. Transactions of the Chinese Society of Agricultural Engineering, No., pp Shi H., Tian J.L. and Liu P.L. (997). Simulative Experiment Research on Relation of Slope and Gully Erosion in Small Watershed. Journal of Soil and Water Conservation, No.3, pp

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