1. Applied Hydrology Infiltration
Prof. Ke-Sheng Cheng
Department of Bioenvironmental Systems Engineering
National Taiwan University

2. Water Distribution in the Soil
(Unsaturated zone)
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

3. The capillary zone lies above the lower saturated layers.
The soil water zone begins at the ground surface and extends downward, encompassing the root layers. During periods of rainfall (or other water application such as irrigation), this area may become saturated. It is otherwise in an unsaturated state, part of the soil pores are filled with air.
The intermediate zone extends down to the capillary fringe. It is unsaturated except during periods of extreme precipitation.
The capillary zone lies above the lower saturated layers.
The saturated zone has all pores filled with water (surficial aquifer).
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

4. Subsurface water appears as hydroscopic, capillary, or gravitational water.
Hydroscopic and capillary water are held by molecular forces in thin films around soil particles.
Hydroscopic water is essentially unavailable.
Capillary water results when more water is available filling gaps between soil particles but in a discontinuous fashion. Capillary water can be in direct connection with groundwater or in isolated pockets.
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

5. Soil moisture at some depth in the intermediate layer may not change with time.
In humid or well-irrigated areas, field capacity (FC), the maximum amount of water the soil can hold against gravity, is a good moisture assumption for this layer.
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

6. Definition of soil moisture content
max
=s
(Saturated soil moisture content)
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

7. Soil Moisture Profile 12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

8. Infiltration rate and infiltration capacity
Infiltration rate: the actual rate of movement of water through soil surface (e.g. cm/hr, inch/hr)
Infiltration capacity: the maximum rate of infiltration when there is an excess supply of water at the surface.
Infiltration capacity is influenced by soil type and initial soil moisture content primarily.
Factors affecting infiltration rate:
SMC
landcover
soil type (hydraulic connectivity)
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

9. Typical soil moisture distribution profile during the downward movement of water.
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

10. Modeling the Infiltration Process
Horton's model for infiltration capacity
The Horton's equation assumes the soil surface is saturated at all time, i.e. excess water at surface.
The value of fc is equivalent to the saturated hydraulic conductivity.
The Horton’s infiltration model is used for modeling point-scale infiltration capacity.
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

11. Horton’s infiltration model
Actual infiltration rate vs infiltration capacity
What is the real infiltration rate at time t1?
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

12. Ponding time Case A. Excess water at surface (ponded infiltration)
(fi*=infiltration capacity, fi=actual infiltration rate)
Case B. Non-ponding infiltration
until ponding at surface occurs.
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

13. 12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

14. 12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

15. 12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

16. Assume a constant rainfall intensity i (fc < i < fo)
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

17. SCS Curve Number (CN) method
The SCS CN method is proposed for infiltration rate estimation in watershed-scale.
P=Pe+Ia+Fa
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

18. P: total rainfall depth of a storm
Pe: total depth of direct runoff, or excess rainfall (inch)
Fa: depth of water retained in the watershed after runoff begins (i.e. the amount of infiltration after runoff begins) (inch)
Ia: initial abstraction before ponding (no runoff occurs)
S: potential maximum retention after runoff begins (inch)
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

19. Initial abstraction (Ia) is all losses before runoff begins
Initial abstraction (Ia) is all losses before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evaporation, and infiltration. Ia is highly variable but generally is correlated with soil and cover parameters.
Through studies of many small agricultural watersheds, Ia was found to be approximated by the following empirical equation:
Ia =0.2S
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

20. CN=100  S=0, Ia=0 CN<100  S>0, Ia>0
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

21. SCS CN method Initial abstraction 12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

22. 12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

23. 12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU

24. Introduction of NRCS TR-55 model
12/6/2018
Lab for Remote Sensing Hydrology and Spatial Modeling, Dept. of Bioenvironmental Systems Eng., NTU