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,
712100, Shaanxi，China( E-mail:[email protected])
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, 2005, 2006).
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, 1952; Chery,
1965; Grace and Eagleson, 1965; Zhou Wende, 1969; Zhu Xian, 1957; Jiang et al., 1994; Yuan et
1
al., 2000; Shi et al., 1997). Based on new views about rainfall and runoff control through
rainwater use and simulative experiment theories (Gao, 2005,2006), 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 0.3417 km2. Its gully density is 3 - 4
per km2. The length of the watershed is 0.903 km, and the widest part of the watershed is 0.723
km, with an average width of 0.52 km. The watershed shape coefficient is 0.52 and the maximum
relative elevation is 190 m. The soil erosion modulus is 9,000 t/ (km2.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
1) under the condition of similarity of geometric, rainfall, flow, sediment movement, and bed
deformation being met (Gao, 2005, 2006). 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 0.025 mm, and 98% of the silt less than 0.1 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
λd =
λ1l/ 4 λ1ν/ 2
λ1ρ/ s2−ρ
(1)
ρ
where λd is the grain scale, λν is kinematic viscosity coefficient scale,
λρ
s −ρ
ρ
is the scale of silt
submerged weight. For natural soil, if the geometric scale is 100, λd = 3.16, d50m= 0.025/3.16 ≈
0.008 mm. Equation (1) provided a reference to select the soil for the model. Figure 1 showed
the calculated and actual adopted model soil graduation when the model soil was natural soil and
the geometric scale was 100.
The starting similarity requires
λu = λu
0
(2)
where λu0 and λu is the scale of the competent velocity and flow velocity. According to Hazen’s
data on overland flow (Gao,2005; Liu and Wu 1997), Fig. 2 gave the relationship between
competent velocity and grain size. From the figure, it is clear that when d50 = 0.025 mm in the
prototype, the competent velocity (v0y) is 0.08 cm/s; but when d50 = 0.00759 mm in the model, the
2
competent velocity (v0m) is 0.0083 cm/s. So,
λu
0
= 0.08/0.0083 ≈ 10, which satisfies the similarity
requirement.
Table 1. Main model scales
Scale Name
Scale Notation
Scale Value
Plane scale
λl
100
Vertical scale
λh
100
Rainfall intensity scale
λi = λ v = λ1l / 2
10
Rainfall amount scale
λ P = λi λt
33.3
Rainfall time scale
λt
3.3
Velocity scale
λv = λ1l / 2
10
Water flow amount scale
λQ = λ5l / 2
100000
Coefficient of roughness
scale
λ n = λ1l / 6
1.47
Flow time scale
λt = λ1l / 2
10
Suspension movement
similarity
λ1l / 4 λν1 / 2
λd = 1 / 2
λ ρ −ρ
Competent velocity scale
λv = λv = λ1l / 2
10
Sediment content scale
λs
3
Deformation time scale
λ t'
3.3
Sediment transporting ratio
scale
λG
300000
Soil water content scale
λθ
1
Infiltration rate scale
λ f = λ1l / 2
10
Geometric similarity
Rainfall similarity
Flow movement similarity
Sediment movement
similarity
Soil water similarity
1
'
3.16
s
ρ
o
3
10
V 0（cm/s）
P(%)
100
90
80
70
1
0.1
60
Antitype Diameter
50
0.0 1
40
Model Demand
30
0 .00 1
20
Real Select
0.000 1
0.001
10
0
0.001
0.01
0.1
Figure 1 Soil Grain Grading Curves
0.1
1
D（ m
1
D(mm)
0.01
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
1.0 t/ km2 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 140 – 150 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 1998 and 1999 were, respectively, 571.4 mm and 494.6 mm, and average number of
eroding rainfall events were, respectively, 7 times and 6 times. Thus, the average eroding rainfall
amount was 141 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 = 8000t - 9000t was an
4
annual prototype erosion amount, and Iy = 1.14 mm/min (see Fig. 3) was an average eroding
rainfall intensity of 70% guaranteed rate during 10 minutes, and Ty = Py/Iy = 150/1.14 ≈ 131.6 min
was an average eroding rainfall time. Thus, the model rainfall intensity of Im = 0.114 mm/min
and average eroding time of Tm = Ty/λt1 =131.6/3.3 ≈ 40 min.
Rainfall Intensity (mm/min)
4.5
10,000 years
1,000 years
4.0
500 years
100 years
3.5
50 years
3.0
20 years
10 years
2.5
5 years
2 years
2.0
1.4 years
1.5
1 year
1.0
0.5
0.0
10
100
1000
Ti me ( mi n)
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 1).
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.
(1) 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, 2005).
(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 0.114 mm/min of rainfall
intensity, and 40-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
180
6
160
140
5
120
100
80
4
3
60
2
1
Dischage
40
Sediment
20
0
0
0
1
T（h）
2
3
P ( %)
200
7
S（kg/ m3）
Q（m 3/ s）
annual erosion amounts were close to that of the prototype. That is, the model could reflect the
reality of hydraulic erosion in the prototype.
100
90
80
70
60
50
40
30
20
10
0
0.0001 0.001
Model
Field 6
Field 5
Field 4
Field 3
0.01
0.1
1
10
D (mm)
Fi
4 h
h f Di h
Figure 4 The Graphs of Discharge
And Sediment Concentration
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 13, 2003) 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)
170
140 - 150
Conflux time (h)
0.3
0.2
Average conflux velocity (m/s)
0.84
1.25
Maximum flow amount (m3/s)
Annual erosion amount (t/km2)
5.00 - 6.39
2,900
5.00 - 6.30
2,800 - 3,100
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 10% 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
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 2003)
Table 3. Equivalent rainfall intensity processes of the model and prototype
Rate of the total area of the watershed (%)
Rainwater catchment type
Area(km2)
Cellar
0.060
18%
Terraced field
0.162
47%
CONCLUSIONS
Based on the above analysis, the following conclusions can be made:
(1) 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 30 minutes in a certain guaranteed rate as the experimental rainfall intensity.
(3) The verifying experiment results show that when the model’ geometric scale was 100, 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.
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ACKNOWLEDGEMENT
The research was financially supported by the Chinese 11th Five-Year National Key Technology R&D Program
Plan of rainfall and runoff controlling and efficient utilizing technology on sloping surface (2006BAD09B01) and
engineering mode of rainwater safe harvesting for single family in rural area of north plain (2006BAD01B04-02).
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