Research on Unsaturated Flow Model in the Plain of the Huaihe River Basin
Ding Feng1 Peng Shunfeng2(1Anhui Hydrological Bureau 2Huaihe River Commission
No.1, Shuguang Road, Hefei, P. R. China)
df8182@163.com
Abstract:
It is crucial important to understand the water cycle in the plains of the Huaihe River Basin not only forforecasting flood, drought and water demand, but also for making short period water withdrawal plan
and long period water development scenarios. In th plains, the unsaturated zone played an importantrole in the water transfer among precipitation, surface water, groundwater, soil water and evaporation
due to its linkage position among the water cycle, so the unsaturated flow was important for coupling
surface water model and groundwater numerical model. The paper focused the general dynamical
unsaturated flow equation, Richard equation, and applied a numerical model to calculate the soil
moisture process. The model was applied at Wudaogou Hydrological Experimental Station (WHES).
The model was divided into for layers, with a thickness of 5cm, 20cm, 30cm and 35cm from top tobottom respectively. The hydrological data measured at WHES was used to calibrate and verify themodel. The measured vertical average soil moisture process was used to evaluate the goodness of themodel. The result showed that the simulated vertical average soil moisture process was similar to thatof measured, so the model was effective for modeling soil moisture process. However, there were long
way to go to apply the model to natural basin due to its complicated heterogeneous and anisotropic soil
property, crops and vegetation distribution, geomorphology, and etc.

1 Introduction of the research
1.1 The Natural Condition of the Huaihe River Basin
The Huaihe River was originated from the Tongbo Mountain of Henan, flowed through Anhui andJiangsu. The total basin area was 270,000km2, out of which two thirds was of the plain or flooded low
land, and the others was mountainous. Corresponding to 15.5*106 ha tillage and 0.165*109 population(the Huaihe River Commission, 2004), the water resources were scarce the in HRB, and its spatial andtemporal distributions were uneven. Nowadays, due to the uneven spatial and temporal distributions of
the water resources and increasing water withdrawal, the conflict between water uses and water supply
was becoming serious in the plains of HRB, and the water resources development was becoming
difficult. However, in the plains, the geomorphology was flat, and the quaternary sediments layer was
thick, so the soil layer supplied good condition for water transfer between surface water, groundwaterand unsaturated water. The transfer relationships were crucial important for flood forecasting, making
reasonable water resources development scenarios and forecasting water uses. In the water transfer
processes, the soil water played a very important role. Therefore, it is necessary to build a model tosimulate soil water change process.

1.2 Objective of the Research
The research objective of the paper was to build a dynamical unsaturated zone flow model. The model
was calibrated and verified by the hydrological data measured in the Wudaogou HydrologicalExperimental Station. After simulating the soil water processes, the annual water budget was analyzed.
Then the general relationships were built via the water budget analyses, such as the relationships
between groundwater evaporation and groundwater head depth, between annual land evaporation andpotential evaporation, between annual precipitation and surface water, between precipitation andgroundwater recharge from precipitation, and etc.

1.3 Research Station and Data Source
The research focused on the data measured at Wudaogou Hydrological Experimental Station (WHES),
which was located in the north of Bengbu, the west of the railway from Beijing to Shanghai, 30km to
downtown of Bengbu roughly. WHES was a large integrated hydrology and water resources
experimental station, with an area of 1.4 ha. Groundwater regime observation set consisted of 62
lysimeters and a large field basement with a diameter of 1100m, a depth of 65m. The runoff
experimental field consisted of 3 overlap catchments, with the largest area of 136 ha, the middle area of
10 ha, and the smallest area of 0.3ha. There were 16 pumping wells and 2 automatic groundwater headmeasured wells in the experimental field.
The long-term annual precipitation, potential evaporation, average groundwater head depth were866mm, 1103mm and 1.14m. The soil was sandy clay loam. The soil moisture constants weremeasured at WHES, and the result was shown in the table 1.1.

Table 1.1 The soil moisture constants measured at WHES
Depth
（cm）

Saturated

Field capacity

water（volume %）Capillary

Wilting point

Moisture absorption

break point（volume %）point（volume %）（volume %）content(volume %)
0-20

39.6

30.7

19.5

14.8

7.4

20-40

31.0

25.8

18.1

14.6

7.3

40-60

29.6

26.5

18.6

14.4

7.2

60-80

28.8

25.2

17.6

15.8

6.9

80-120

27.5

24.8

17.4

15.6

7.8

120-200

27.4

24.6

17.3

16.4

8.2

The data observed at WHES applied in the research included followings:(1) The daily meteorological data from 1994 to 2000, such as precipitation, air temperature, groundtemperature, air humidity, wind velocity, solar radiation and water surface evaporation measured
with evaporation pan.
(2) The daily soil moisture content in the depths of 5, 25, 35, 45, 55, 70 and 90cm from 1994 to 2000.
(3) The daily groundwater head (and its depth).
(4) Crops type, soil property.

2 Unsaturated Flow Model
According to Richard equation, the dynamical unsaturated flow process can be simulated by equation
2.1.

∂θ∂∂θ∂K= (D ) ++ F (t ,θ )
∂t ∂z
∂z∂z

(2.1)

where K and D were conductivity and diffusion coefficient of unsaturated zone respectively, and they
were functions of soil moisture, θ. Here the equation 2.2 and 2.3 were applied to estimated K and D.

K (θ ) = K s (θ / θ s ) 2b +3(2.2)

D(θ ) = K (θ )(∂Ψ / ∂θ )

(2.3)

Ψ (θ ) = Ψs /(θ / θ s ) b(2.4)

where， K s and Ψs were soil conductivity and potential of saturated zone respectively, and b was aempirical coefficient, and they were function of soil property (Cosby et al., 1984). F (t , θ ) weresinks and sources, such as precipitation, evaporation, runoff, and etc.
In the research, the soil layer was from land surface to 90cm depth. In the range, the soil moisturemight be calculated with four layers model, so the Richard equation might be rewritten to equation 5 ~
8 (Chen F., et al. , 1994a, b. Chen Xi, et al, 2004).

d1∂θ 1 ∂θ = −D 
 − K 1 + Pd − R s − E u − E T 1∂t ∂z 1(2.5)

d2∂θ 2 ∂θ  ∂θ = D + K1 − K 2 − E d − ET 2
 − D∂t ∂z  2 ∂z 1(2.6)

d3

∂θ 3
 ∂θ  ∂θ = D + K 2 − K 3 − ET 3
 − D∂t ∂z  3 ∂z  2(2.7)

d4

∂θ 4
 ∂θ  ∂θ = D + K3 − K4
 − D∂t ∂z  4 ∂z  3（2.8）

where d1, d2, d3 and d4 were the thickness of layer 1 to 4 from the land surface to 1m depth soilrespectively, K1, K2, K3 and K4 were the conductivities of layer 1 to 4 respectively, Pd was effect rain,
runoff Rs was surface runoff, Eu and Ed were soil evaporation via shallow and deep layers, ET1, ET2
and ET3 were evapotranspiration via the leaves and root of vegetation in layer 1 to 3 respectively.
If groundwater head depth was deeper than the threshold of groundwater evaporation depth, thegroundwater evaporation would be neglected, then the lowest layer flow equation can be written toequation 2.9.

d4∂θ 4 ∂θ = D +K3 −K4∂t ∂z  3(2.9)

After replace the differential equations with difference equations approximately, Equation 2.5 ~2.9 can be solved from an initial soil moisture condition measured in advance, and the soil
moisture series might be calculated for every layer.

3 Model Application at Wudaogou Hydrological Experimental Station
The daily data of soil moisture (θ), precipitation series(P), runoff (Rs), groundwater head and its depth,
and groundwater evaporation (Eu, Ed) measured at WHES were applied to the model to simulated soilmoisture process. The whole simulated soil layer was from land surface to 90cm depth, and it wasdivided into four layers with a thickness of 5cm, 20cm, 30cm and 35cm from top to bottom
respectively. The parameters about the soil property tested at WEHS were applied in the model, andthey were shown in Table 1.1.
After input the daily precipitation, open water evaporation, groundwater depth and the parameters boutthe soil property measured at WHES, the vertical average soil moisture from land surface to 90cm
depth might be calculated. The results were shown Figure 3.1.

Figure 3.1 the soil moisture process of 1997

4 Conclusions and Recommendations
The result showed that the unsaturated model might be applied to simulate the soil moisture process at
WHES. The simulated series were similar to that of measured.
The unsaturated model was the core of the four kinds of water transfer model, so it might be applied tolink the surface water model and groundwater dynamical model. Therefore, an integrated model might
be built in the next step research. However, it was very difficult to apply the unsaturated model innatural basin due the constraints of land surface property data, such as geomorphologic data, vegetationand crops cover data, soil grain data, irrigation data, conductivity in different point and direction, andso on. Another constraints for the model application was the soil property parameters changed rules,
such as conductivity, K, soil potential,Ψ, and diffusion coefficient, D, changed processes with the soil
moisture. For understand the relationships mentioned above, a lot of experiment needed to be done.
Finally, the paper only took some consideration on vertical flow of soil moisture. If the model wasapplied to natural basin, the horizontal direction flow had to be thought over.