- Email: [email protected]
Integrated process studies and modelling simulations of hillslope hydrology and interflow dynamics using the HILLS model
Environmental Modelling & Software 14 (1999) 153–160
Integrated process studies and modelling simulations of hillslope hydrology and interflow dynamics using the HILLS model W.-A. Flu¨gel a
a,*
, R.E. Smith
b
Geographisches Institut, Friedrich-Schiller-Universita¨t Jena, Lo¨bdergraben 32, D-07743 Jena, Germany b ARS-USDA, Foothills Campus CSU, Fort Collins, CO 80523, USA
Abstract Interflow is of major importance for runoff generation and groundwater recharge in mountainous and hilly catchments. However, integrated process studies and model simulation of the hydrological dynamics of hillslope drainage by interflow are still scarce. Therefore, such a study has been carried out on the Kiefer-Hang test hillslope in the catchment of the Sieg River, located in the middle mountain range of the Rheinische Schiefergebirge in Germany. The overall objective was to analyse and to simulate soil moisture, interflow and groundwater recharge with a high resolution in time and space. For the simulation study the two-dimensional HILLS numerical hillslope model was selected, parameterized and calibrated by detailed field surveys. The simulations were undertaken using measured hydro–meteorological data with 5-minute intervals. The process analyses of the study revealed the hydrological dynamics of the test slope, and the results of the simulations proved the ability of the HILLS model to simulate interflow. However, some extreme storms and dry weather periods were simulated poorly because: (i) the soil-physics is simplified in the model as compared to the complex structure of the test slope, and (ii) HILLS in its present version does not account for feedback from the level of the adjacent Breidenbach Creek. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Interflow modelling; Hillslope hydrology; Groundwater recharge; Runoff generation
Software availability Name of software: Developer: Year first available: Contact address:
Hardware required:
Software required: Program language: Program size: Availability and Cost:
* Corresponding author. [email protected]
HILLS Dr R.E. Smith 1990 Dr Roger Smith, ARSUSDA, Foothills Campus CSU; Fort Collins, CO 80523, USA, E-mail: [email protected] IBM compatible 386 or higher, 8 MB memory (RAM) DOS FORTRAN90 541 KB Public Domain software
Fax:
⫹
49-3641-948852;
e-mail:
Availability of dataset:
(PD-software) freely available From corresponding author
1. Introduction 1.1. Interflow and hillslope hydrology Interflow was defined by Flu¨gel (1993) as: “the portion of rainfall which infiltrates on the hillslope and percolates through the soil till it reaches less permeable layers on which it flows in saturated and unsaturated conditions downslope, seeping either directly or via the groundwater aquifer into the river”. Historically the importance of interflow for the generation of river runoff has been pointed out by Kirwald (1955) using hydrograph analyses. Hewlett and Troendle (1975) discussed the seasonal hydrological dynamics of a hillslope using the variable source area concept (VSAC). A comprehensive publication about hillslope hydrology and modelling
1364-8152/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 4 - 8 1 5 2 ( 9 8 ) 0 0 0 6 6 - 8
154
W.-A. Flu¨gel, R.E. Smith / Environmental Modelling & Software 14 (1999) 153–160
approaches was presented first by Kirkby (1978) and later by Anderson and Burt (1990). Detailed field studies on interflow dynamics were carried out by Flu¨gel (1978) on a test slope near Heidelberg in Germany. Flu¨gel (1993) introduced the concept of the dynamic total storage for his interflow studies, which he defined as a vertical section of the valley floor adjacent to the hillslope comprising the saturated aquifer zone and the unsaturated rooted zone. He thereby established a conceptual link between the hillslope hydrology, groundwater recharge in the valley floor, interflow seepage from the adjacent hillslope and river runoff. He also showed that interflow depends on the stratigraphic structure of the hillslope and is generated on top of less permeable layers in the slope. The unsaturated zone controls the interflow dynamics and a minimum soil water storage usually must be reached before interflow occurs. 1.2. Simulation of hillslope hydrology
Fig. 1.
Representation of a hillslope segment in the HILLS model.
Physically based simulation of hillslope hydrology must account for the heterogeneity of the hillslope system and has to comprise the components of the hydrological cycle (Chorley, 1978). For a hillslope they have been mathematically described by Freeze (1978). To the knowledge of the authors there are, however, only few hillslope hydrology models available for testing. Such models are HYDRUS (Kool and van Genuchten, 1991), HILLFLOW (Bronstert, 1994) and HILLS (Hebbert and Smith, 1992). HYDRUS also simulates solute transport, while HILLFLOW and HILLS concentrate on the hillslope hydrology. The former model makes use of the Richards equation for saturated–unsaturated water flow, and HILLFLOW and HILLS apply the kinematic wave approach for routing overland flow and unsaturated flow. Unlike the other models, HILLS can also approximately simulate the groundwater dynamics in the valley adjacent to the slope. This was the crucial point for choosing HILLS for this study. The conceptual design of HILLS can be seen in Fig. 1 showing the hillslope design within HILLS and in Fig. 2 showing the model organisation. 1.3. Test hillslope Kiefer-Hang The test hillslope named Kiefer-Hang is located approximately 30 km north-east of Bonn. As shown in Fig. 3 it is located within the catchment of the River Bro¨l (A ⫽ 216 km2), which is a tributary of the River Sieg in the middle mountain range named Rheinisches Schiefergebirge in Germany. The River Sieg has its confluence with the River Rhein north of the city of Bonn. The climate of the Bro¨l catchment is oceanic influenced and described by the annual means: (i) precipitation of 1039 mm; (ii) potential evapotranspiration of 536 mm; and (iii) air temperature of 8.5°C. Land use of the catch-
Fig. 2. Flow chart of water transfer within the hillslope system as represented by the HILLS model.
ment consists of 52.6% grazing pastures, 33.8% coniferous and deciduous forest, 2.7% agricultural farmland and 10.9% settlements (Flu¨gel, 1995). The slope presently is used as meadow with two cuts a year and no grazing between these cuts. The test slope has a north-north-west aspect, with a cross profile length of 174 m, and its cross profile (Fig. 4) is representative for the slopes within the Bro¨l catchment: a convex top changes into an even gradient slope
W.-A. Flu¨gel, R.E. Smith / Environmental Modelling & Software 14 (1999) 153–160
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the groundwater is close to the surface and drains directly towards the adjacent Breidenbach Creek. 1.4. Instrumentation and database The instrumentation of the test slope accounted for the spatial heterogeneity of the hillslope in terms of soil distribution and the data input requirements of the HILLS model. Overall seven field stations K0–K6 have been installed, six of them shown in Fig. 4 along the cross profile of the slope. They were equipped with pressure transducer tensiometers at 15, 30, 60, 90 and 120 cm depths (if the soil layer depth permitted) connected to dataloggers. In station K5 and K6 groundwater levels were recorded, as well as the water level in the adjacent Breidenbach Creek near station K6. The weather station was installed at K0 and recorded windspeed, solar radiation, soil and air temperatures and relative humidity at 5-minute intervals.
2. Results Fig. 3. Location of the Kiefer-Hang hillslope within the catchment of the River Sieg in the Rheinische Schiefergebirge, Germany.
Fig. 4. Cross profile and location of the tensiometer stations along the Kiefer-Hang hillslope. (Note: K3 is not within this profile.)
which in turn changes into a convex footslope adjacent to a small and flat valley floor towards the Breidenbach Creek. This test section should allow a future regionalisation of the results within the Bro¨l catchment using the concept of Hydrological Response Units (HRUs) presented by Flu¨gel (1995). The underlying geology is Devonian shale which is almost impermeable for deep percolation losses. A cross profile drilling revealed a step within the surface of the weathered bedrock (Fig. 4), which was filled with eroded soil washed in from the upper hillslope and holds a perched watertable during most of the year. At the upper and middle part of the slope former agriculture has resulted in eroded brown soils. The foot slope is covered by hydromorphic colluvial brown soils, which change to groundwater influenced gleyic soils towards the valley floor. Within the small valley floor
2.1. Data analyses 2.1.1. Preprocessing The 5-minute continuous data time series of the three hydrological years (1994 till 1996) were preprocessed by checking homogeneity and consistency. Unreliable tensiometer readings during frost periods in the winter were replaced by missing values. The missing values were replaced (if possible) by estimated values calculated from highly correlated regression models using reliable data from other stations. Although not all gaps could be filled by such regression calculations the project database is considered to be highly reliable for the process analyses and the simulation exercise. The period of November 1994 to October 1995 has been used in this paper for analysing the hydrological dynamics of the test slope and for carrying out the simulation runs. 2.1.2. Climate Using hourly means of the weather data from K0, potential evapotranspiration (ETP) was calulated by means of the Penman–Monteith equation (ASCE, 1990), which was included into the HILLS model for this simulation study. According to Flu¨gel, 1978, 1993) precipitation and the actual evapotranspiration (ETA) are the major water balance input and output, controlling the generation of interflow and the hillslope drainage. ETA in turn depends on the plant transpiration activity which is expressed in HILLS as a function of the actual soil water storage. The precipitation in the hydrological year 1995 amounts to 1176 mm with a daily maximum of 58.2 mm in December 1994. From this annual total altogether 63% (746 mm) was received on the test slope
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during the winter half-year between November 1994 and April 1995. ETP was calculated at about 672 mm, of which the vegetation period between May and October 1995 accounts for 80% (537 mm). The distribution of rainfall and ETP for the hydrological year 1995 is given in Fig. 5. The partial water balance can be described as follows: (i) During November 1994 to April 1995 the slope received 746 mm of precipitation. ETP was calculated to about 135 mm, which gives a positive balance surplus of 611 mm (81.9%). (ii) In the summer (May to October 1995) altogether 430 mm of rainfall was recorded, which gives a negative balance of 107 mm if compared with the calculated ETP of 537 mm for this period. 2.1.3. Soil moisture, groundwater dynamics and runoff The interactive dynamics of the unsaturated and saturated zone can best be studied during the winter halfyear. This is shown together with the runoff of the Breidenbach Creek in Fig. 6 using 5-minute data for a 4-day time period in January, 1995. The dynamics of soil tension observed by the tensiometers at station K6 were also recorded in a similar way at the other five stations of the Kiefer-Hang although the actual tension values measured along the slope differed. Water levels recorded at K6 in the Breidenbach Creek were transferred into discharge given as L/s by a calibration curve, which was established from measured cross-profile flow velocities at different water levels. Fig. 6 shows the following elements of the hydrological dynamics of the test slope: (i) As the soil layer approaches saturation, even smaller rainfall events result in a rise of groundwater in the valley floor and runoff in the Breidenbach Creek. (ii) The soil moisture dynamics—expressed as soil tension in different depths—is similar during the three major storms. For the storm of January 28 it shows that: 쐌 During the storm infiltrating rainfall decreased the soil
Fig. 6. Rainfall, soil tensions, groundwater depths and runoff of the Breidenbach Creek at the stations K5 and K6 between 26–29 January 1995.
tension to near zero. This decrease occurred with a certain delay according to the installation depth of each instrument. At 90 cm depth some positive soil water pressures were recorded because of the groundwater head at the tensiometer cell. 쐌 At K6 infiltrating water percolated through the soil to the groundwater, bringing its level to about 40 cm below the surface of the valley floor. Groundwater is rising above the tensiometers and as a result these instruments are measuring positive soil water pressure instead of soil tension. 쐌 On the slope the percolating water reached the underlaying impermeable Devonian shale, generating a saturated zone and an interflow hydrograph. The latter was observed at station K5 when it reached the footslope zone. 쐌 After the end of the storm the soil started to drain again by gravity, providing the recession limb of the interflow hydrograph. (iii) Groundwater dynamics and runoff in the Breidenbach Creek reflect the rainfall and interflow inputs in the following way:
Fig. 5. Measured daily precipitation (P) and calculated potential evapotranspiration (ETP) during the hydrological year 1995.
쐌 Due to surface runoff from farm roads the runoff in the Breidenbach Creek responds almost instantly to the rainfall and slightly earlier than the rise of groundwater level in the valley floor at K6. 쐌 Comparing the rising limbs of the hydrographs from K6 and in the Breidenbach Creek, a steplike break can be seen during the rise. This break coincides with
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the delayed rise of the groundwater level at station K5 which was the result of the interflow hydrograph from the hillslope. 쐌 Further rise of the runoff hydrograph after this break obviously is caused by interflow from the test slope which is moving through the almost filled groundwater aquifer of the valley floor towards the Breidenbach Creek. 쐌 Comparing the similar dynamics of the hydrograph recessions of Breidenbach runoff and groundwater at K5 reveals that after the storm the runoff in the Breidenbach Creek is mainly controlled by interflow, draining the slopes of the catchment. This hydrological dynamics was frequently observed and is a typical response of the “dynamic total storage” as described by Flu¨gel (1993) in the valley floor and the footslope zone, which is jointly controlled by infiltration, groundwater recharge, interflow hydrographs and runoff contribution. 2.1.4. Hydrological dynamics of the Kiefer-Hang hillslope The results of the field survey, the climatic balance and the process studies discussed above reveal the hydrological dynamics of the Kiefer-Hang hillslope system as follows: (i) During the winter half-year the precipitation surplus (i.e. in 1995 about 611 mm) was infiltrating into the soil recharging the soil water storage. The soil infiltration capacity of such a grassland slope always exceeds the rainfall rate and consequently surface runoff on the slope was never observed. This is in line with results reported by Flu¨gel, 1978, 1993 from a test slope with similar vegetation, soil texture and topography near Heidelberg, Germany. (ii) At the beginning of the storm, infiltrating precipitation accumulates first in the soil layer and a wetted region grows from the soil surface. As this zone becomes wetter, infiltrating water moves more rapidly downwards, generating a saturated zone above the underlying impermeable Devonian shale. If macropores (e.g. from earth worms) are present water can also flow directly to the surface of the underlying bedrock. (iii) Due to the slope gradient interflow is generated within this saturated layer and moves laterally downslope along this layer towards the valley floor. This analysis is confirmed by the hydromorphic pseudogleyic character of the soils above the Devonian shale which was found all along the hillslope during drilling (Fig. 4). (iv) Interflow moves from the slope through the adjacent groundwater system, contributing to the runoff of the Breidenbach Creek. A variable time delay might be caused by an intermediate storage of the interflow as
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perched groundwater in the bedrock ditch shown at station K2 in Fig. 4. (v) During the summer half-year ETP is mostly higher than the rainfall input, as in 1995 when rainfall was 430 mm and ETP was calculated to about 537 mm. Precipitation which is not intercepted, infiltrates and is stored within the soil for a later use by evapotranspiration. Periods with groundwater recharge and interflow are rare and restricted to wet summerly conditions such as reported by Flu¨gel (1993). From this analysis it can be concluded that the hillslope drainage, the generation of interflow and its runoff contribution is mainly restricted to the hydrological winter half-year having a positive water balance. During the summer half-year the water balance is negative and the discharge of the Breidenbach Creek is established by occasional storm runoff and groundwater seepage during dry weather periods. 2.2. Modelling 2.2.1. Representing the hillslope The soil of the slope was differentiated into two layers, of which the lower one, the Devonian shale was regarded as being almost impermeable. The hillslope geometry (Fig. 1) can be described within HILLS by up to 20 nodes along the hillslope. At each node the depth of the surface soil and the slope gradient must be defined. HILLS also permits the identification of two checkpoints at which the dynamics of the perched watertable are reported. These points are essential for the calibration of the model, and the two tensiometers at stations K5 and K6 with recording groundwater level devices were used. 2.2.2. Parameterization A method of estimating hillslope parameters for the HILLS model was given by Hebbert and Smith (1990). The major physical parameters controlling the soil moisture dynamics within HILLS are: (i) soil volumetric saturated water content; (ii) saturated vertical hydraulic conductivity; (iii) an anisotropy factor, which is a quotient relating horizontal to vertical hydraulic conductivity; (iv) the pore-size distribution index; (v) the capillary fringe height; and (vi) the depth of the perched watertable at the bottom end of the hillslope. Disturbed and undisturbed samples collected at station K2 and K6 were analysed to parameterize the hillslope and the valley floor respectively. Basic soil physical parameters obtained from undisturbed samples from a soil pit in station K6 in the foot slope of the cross-section are listed in Table 1. For the description of the soil horizons the German soil classification has been used. The anisotropy will govern the value of Kh, and according to the soil
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Table 1 Soil physical parameters from tensiometer station K6 used to parameterize the HILLS model Soil horizon/depth Ah 0-15 cm Bv 16-30 cm Go 31-60 cm Gr > 60 cm
PV (%)
FC (%)
BD (g/cm3)
Ku (m/s)
54.1 54.1 43.0 41.7
49.3 46.8 39.3 35.6
1.12 1.10 1.46 1.50
1.55E-05 3.80E-05 3.46E-06 2.23E-07
Ah ⫽ humic A; Bv ⫽ weathered B; Go, Gr ⫽ groundwater influenced G; PV ⫽ total pore volume; BD ⫽ bulk dry density; Ku ⫽ saturated conductivity; FC ⫽ field capacity.
survey values between 1.0 and 1.2 have been chosen. The divergence angle ␦ used to describe a converging or diverging hillslope shape was set to zero, since the Kiefer-Hang hillslope is a relatively non-converging slope. Data from the tensiometer station K6 in the valley floor were used to validate the simulations of soil moisture and groundwater dynamics. 2.2.3. Groundwater dynamics The ability of the HILLS model to simulate the groundwater dynamics in the valley floor will be discussed for different time periods and considering the seasonal hydrological dynamics described above. All simulations are based on the measured 5-minute rainfall data and the corresponding groundwater dynamics, (note that an input interval of 5 min is not required by the HILLS model). The simulation results for a period of the winter half-year 1995 are shown in Fig. 7 and reveals: (i) Except for the storm in January 1995 the measured groundwater dynamics at tensiometer station K6 is reproduced reasonably well. Deviation between measured and simulated data can be explained by the simplification of the slope’s soil physical heterogeneity within the HILLS model. (ii) During heavy storms in January and February 1995 the HILLS model generates a water level which is
Fig. 7. Measured and simulated groundwater depths at station K6 between 2 January to 30 April 1995, based on 5-minute data intervals.
greater than measured at station K6. This discrepancy might be the result of several simplifications within HILLS: Inadequacy of the Dupuit–Forchheimer assumptions for the lower creek area, vertical heterogeneity of soil porosity, or inadequacy of the use of fixed water level for the downstream boundary condition, rather than a dynamic creek level. The simulations of the 10-month period from 1995 shown in Fig. 8 are also based on 5-minute continuous simulations, but for the presentation daily averages were used. This period comprised different dynamic hydrometeorological conditions, which in the HILLS model could only be described by constant model parameters. The following observations come from these results: (i) The agreement between the measured and simulated groundwater depth improved due to the averaging as positive and negative small time scale deviations were compensated. (ii) During the summer half-year the soils of the test slope lost water by evapotranspiration, and interflow ceased almost completely. In such dry weather conditions HILLS tends to overestimate the groundwater response corresponding to summer storms although the temporal groundwater dynamics was reproduced fairly well.
Fig. 8. Measured and simulation of groundwater depths at K6 between January and November 1995.
W.-A. Flu¨gel, R.E. Smith / Environmental Modelling & Software 14 (1999) 153–160
(iii) Groundwater depths during periods with dominant dry weather conditions, i.e. in August end September 1995 were only poorly simulated by HILLS. The groundwater depth in the valley floor was underestimated probably because the model uses a fixed creek level for a lower boundary condition, and the Breidenbach Creek level is not updated in the model. It also seems that HILLS may not sufficiently account for the capillary rise of groundwater into the overlaying unsaturated zone from where it is used by evapotranspiration. 2.2.4. Runoff contribution by interflow As discussed before the contribution of interflow to runoff generation is of major importance in hilly catchments. The HILLS model simulates interflow seepage heights, given in mm, which are shown together with the measured discharge of the Breidenbach Creek in Fig. 9. Although the measuring units are different the following observations can be made from the comparison of the two time series: (i) Discharge dynamics of the Breidenbach Creek and interflow height simulated by the HILLS model from the test hillslope Kiefer-Hang have an almost identical dynamics, clearly demonstrating the importance of interflow for the runoff generation within this catchment. (ii) A time delay between measured runoff peaks and simulated interflow contribution might be caused from surface runoff generated on farm roads and outflow of interflow in the foothill zone. Both were observed in the winter periods meanwhile overland flow on the hillslope surface was not observed during the total measurement period.
3. Concluding discussion From the interflow study presented in this paper one may make the following conclusions:
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(i) Integrating hydrological process studies and continuous process simulations are a prerequisite to analysis of the hydrological hillslope and interflow dynamics. The concept of the dynamic total storage (Flu¨gel, 1993) could also be applied to the process study of the KieferHang hillslope. (ii) During periods with positive water recharge the HILLS model simulated the observed groundwater dynamics fairly reasonably. Deviations between measured and simulated time series can be explained by the simplified representation of the physiographic heterogeneity of the slope systems in the HILLS model, which cause some deviations from intermediate groundwater storages as observed at station K2 (Fig. 2). (iii) Groundwater level dynamics near the creek was poorly simulated during heavy winter storms and in summer periods with dominant dry weather conditions. Beside the use of a few constant parameters to describe a system which may have complex seasonal changes, the main reason for such a poor simulation is probably the absence of a time variable lower boundary based on the water level in Breidenbach Creek. In addition HILLS evapotranspiration simulation is very simple, and may be inadequate for capillary rise of water from the perched groundwater system for evapotranspiration during summer. (iv) Systems analyses using the measured hydrographs as well as the interflow simulations provided by the HILLS model confirm the importance of interflow for runoff generation on such catchments.
4. Future research direction Based on the discussions above, future research needs related to the problem of interflow should focus on the following subjects: (i) Additional integrated process and simulation studies on this hydrological component should be carried out in test catchments having different climate, geology and physiographic conditions. (ii) Physically based models such as HILLS should describe the complete sytem of the “dynamic total storage” (Flu¨gel, 1993) including the interaction between the aquifer in the valley floor and the adjacent stream.
Fig. 9. Measured runoff (Q) in the Breidenbach Creek and simulated interflow (I) contribution from the Kiefer-Hang hillslope over the period of January to April 1995.
(iii) Results from such integrated studies must be regionalised in distributed catchment models. The concept of Hydrological Response Units (HRUs) developed by Flu¨gel (1995) can be applied for such a regionalisation approach.
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References ASCE, 1990. Evapotranspiration and irrigation water requirements. ASCE Manuals and Reports on Engineering Practice, 70. American Society of Civil Engineers, New York, 323 pp. Anderson, M.G., Burt, T.P., 1990. Process Studies in Hillslope Hydrology. John Wiley and Sons, Chichester. Bronstert, A., 1994. Modellierung der Ablussbildung und der Bodenwasserdynamik von Ha¨ngen. IHW, Univ. Karlsruhe 46, 192. Chorley, R.J., 1978. The hillslope hydrological cycle. In: Kirkby, M.J. (Ed.) Hillslope Hydrology. John Wiley and Sons, Chichester, pp. 1–42. Flu¨gel, W.-A., 1978. Untersuchungen zum Problem des Interflow. Heidelberger Geographische Arbeiten 56, 275. Flu¨gel, W.-A., 1993. Hangentwa¨sserung durch Interflow und seine Regionalisierung Einzugsgebiet der Elsenz (Kraichgau). Berliner Geographische Arbeiten 78, 68–94. Flu¨gel, W.-A., 1995. Delineating hydrological response units (HRU’s) by GIS analysis for regional hydrological modelling using PRMS/MMS in the drainage basin of the River Bro¨l, Germany. Hydrological Processes 9, 423–436.
Freeze, R.A., 1978. Mathematical models of hillslope hydrology. In: Kirkby, M.J. (Ed.) Hillslope Hydrology. John Wiley and Sons, Chichester, pp. 177–226. Hebbert, R.H.B., Smith, R.E., 1990. Hillslope parameter estimation using the inverse procedure. Journal of Hydrology 119, 307–334. Hebbert, R.H.B., Smith, R.E., 1992. User manual for HILLS, Vers. 5.2, Numerical Hillslope Model. Unpublished Manual, US Department of Agriculture, Agricultural Research Service, Water Management Unit, Fort Collins, CO, USA. Hewlett, J.D., Troendle, C.A., 1975. Non-point and diffuse water sources: A variable source area problem. Paper presented on Watershed Management, Utah State University, Logan, ASCE, New York, pp. 21–46. Kirkby, M.J., 1978. Hillslope Hydrology. John Wiley and Sons, Chichester. ¨ ber Wald- und Wasserhaushalt im Ruhrgebiet. Kirwald, E., 1955. U RTV, Essen. Kool, J.B., van Genuchten, M.Th., 1991. HYDRUS Version 3.31. Unpublished Manual, US Salinity Laboratory, Agricultural Research Service, US Department of Agriculture, Riverside, California, USA.
Integrated process studies and modelling simulations of hillslope hydrology and interflow dynamics using the HILLS model W.-A. Flu¨gel a
a,*
, R.E. Smith
b
Geographisches Institut, Friedrich-Schiller-Universita¨t Jena, Lo¨bdergraben 32, D-07743 Jena, Germany b ARS-USDA, Foothills Campus CSU, Fort Collins, CO 80523, USA
Abstract Interflow is of major importance for runoff generation and groundwater recharge in mountainous and hilly catchments. However, integrated process studies and model simulation of the hydrological dynamics of hillslope drainage by interflow are still scarce. Therefore, such a study has been carried out on the Kiefer-Hang test hillslope in the catchment of the Sieg River, located in the middle mountain range of the Rheinische Schiefergebirge in Germany. The overall objective was to analyse and to simulate soil moisture, interflow and groundwater recharge with a high resolution in time and space. For the simulation study the two-dimensional HILLS numerical hillslope model was selected, parameterized and calibrated by detailed field surveys. The simulations were undertaken using measured hydro–meteorological data with 5-minute intervals. The process analyses of the study revealed the hydrological dynamics of the test slope, and the results of the simulations proved the ability of the HILLS model to simulate interflow. However, some extreme storms and dry weather periods were simulated poorly because: (i) the soil-physics is simplified in the model as compared to the complex structure of the test slope, and (ii) HILLS in its present version does not account for feedback from the level of the adjacent Breidenbach Creek. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Interflow modelling; Hillslope hydrology; Groundwater recharge; Runoff generation
Software availability Name of software: Developer: Year first available: Contact address:
Hardware required:
Software required: Program language: Program size: Availability and Cost:
* Corresponding author. [email protected]
HILLS Dr R.E. Smith 1990 Dr Roger Smith, ARSUSDA, Foothills Campus CSU; Fort Collins, CO 80523, USA, E-mail: [email protected] IBM compatible 386 or higher, 8 MB memory (RAM) DOS FORTRAN90 541 KB Public Domain software
Fax:
⫹
49-3641-948852;
e-mail:
Availability of dataset:
(PD-software) freely available From corresponding author
1. Introduction 1.1. Interflow and hillslope hydrology Interflow was defined by Flu¨gel (1993) as: “the portion of rainfall which infiltrates on the hillslope and percolates through the soil till it reaches less permeable layers on which it flows in saturated and unsaturated conditions downslope, seeping either directly or via the groundwater aquifer into the river”. Historically the importance of interflow for the generation of river runoff has been pointed out by Kirwald (1955) using hydrograph analyses. Hewlett and Troendle (1975) discussed the seasonal hydrological dynamics of a hillslope using the variable source area concept (VSAC). A comprehensive publication about hillslope hydrology and modelling
1364-8152/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 4 - 8 1 5 2 ( 9 8 ) 0 0 0 6 6 - 8
154
W.-A. Flu¨gel, R.E. Smith / Environmental Modelling & Software 14 (1999) 153–160
approaches was presented first by Kirkby (1978) and later by Anderson and Burt (1990). Detailed field studies on interflow dynamics were carried out by Flu¨gel (1978) on a test slope near Heidelberg in Germany. Flu¨gel (1993) introduced the concept of the dynamic total storage for his interflow studies, which he defined as a vertical section of the valley floor adjacent to the hillslope comprising the saturated aquifer zone and the unsaturated rooted zone. He thereby established a conceptual link between the hillslope hydrology, groundwater recharge in the valley floor, interflow seepage from the adjacent hillslope and river runoff. He also showed that interflow depends on the stratigraphic structure of the hillslope and is generated on top of less permeable layers in the slope. The unsaturated zone controls the interflow dynamics and a minimum soil water storage usually must be reached before interflow occurs. 1.2. Simulation of hillslope hydrology
Fig. 1.
Representation of a hillslope segment in the HILLS model.
Physically based simulation of hillslope hydrology must account for the heterogeneity of the hillslope system and has to comprise the components of the hydrological cycle (Chorley, 1978). For a hillslope they have been mathematically described by Freeze (1978). To the knowledge of the authors there are, however, only few hillslope hydrology models available for testing. Such models are HYDRUS (Kool and van Genuchten, 1991), HILLFLOW (Bronstert, 1994) and HILLS (Hebbert and Smith, 1992). HYDRUS also simulates solute transport, while HILLFLOW and HILLS concentrate on the hillslope hydrology. The former model makes use of the Richards equation for saturated–unsaturated water flow, and HILLFLOW and HILLS apply the kinematic wave approach for routing overland flow and unsaturated flow. Unlike the other models, HILLS can also approximately simulate the groundwater dynamics in the valley adjacent to the slope. This was the crucial point for choosing HILLS for this study. The conceptual design of HILLS can be seen in Fig. 1 showing the hillslope design within HILLS and in Fig. 2 showing the model organisation. 1.3. Test hillslope Kiefer-Hang The test hillslope named Kiefer-Hang is located approximately 30 km north-east of Bonn. As shown in Fig. 3 it is located within the catchment of the River Bro¨l (A ⫽ 216 km2), which is a tributary of the River Sieg in the middle mountain range named Rheinisches Schiefergebirge in Germany. The River Sieg has its confluence with the River Rhein north of the city of Bonn. The climate of the Bro¨l catchment is oceanic influenced and described by the annual means: (i) precipitation of 1039 mm; (ii) potential evapotranspiration of 536 mm; and (iii) air temperature of 8.5°C. Land use of the catch-
Fig. 2. Flow chart of water transfer within the hillslope system as represented by the HILLS model.
ment consists of 52.6% grazing pastures, 33.8% coniferous and deciduous forest, 2.7% agricultural farmland and 10.9% settlements (Flu¨gel, 1995). The slope presently is used as meadow with two cuts a year and no grazing between these cuts. The test slope has a north-north-west aspect, with a cross profile length of 174 m, and its cross profile (Fig. 4) is representative for the slopes within the Bro¨l catchment: a convex top changes into an even gradient slope
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the groundwater is close to the surface and drains directly towards the adjacent Breidenbach Creek. 1.4. Instrumentation and database The instrumentation of the test slope accounted for the spatial heterogeneity of the hillslope in terms of soil distribution and the data input requirements of the HILLS model. Overall seven field stations K0–K6 have been installed, six of them shown in Fig. 4 along the cross profile of the slope. They were equipped with pressure transducer tensiometers at 15, 30, 60, 90 and 120 cm depths (if the soil layer depth permitted) connected to dataloggers. In station K5 and K6 groundwater levels were recorded, as well as the water level in the adjacent Breidenbach Creek near station K6. The weather station was installed at K0 and recorded windspeed, solar radiation, soil and air temperatures and relative humidity at 5-minute intervals.
2. Results Fig. 3. Location of the Kiefer-Hang hillslope within the catchment of the River Sieg in the Rheinische Schiefergebirge, Germany.
Fig. 4. Cross profile and location of the tensiometer stations along the Kiefer-Hang hillslope. (Note: K3 is not within this profile.)
which in turn changes into a convex footslope adjacent to a small and flat valley floor towards the Breidenbach Creek. This test section should allow a future regionalisation of the results within the Bro¨l catchment using the concept of Hydrological Response Units (HRUs) presented by Flu¨gel (1995). The underlying geology is Devonian shale which is almost impermeable for deep percolation losses. A cross profile drilling revealed a step within the surface of the weathered bedrock (Fig. 4), which was filled with eroded soil washed in from the upper hillslope and holds a perched watertable during most of the year. At the upper and middle part of the slope former agriculture has resulted in eroded brown soils. The foot slope is covered by hydromorphic colluvial brown soils, which change to groundwater influenced gleyic soils towards the valley floor. Within the small valley floor
2.1. Data analyses 2.1.1. Preprocessing The 5-minute continuous data time series of the three hydrological years (1994 till 1996) were preprocessed by checking homogeneity and consistency. Unreliable tensiometer readings during frost periods in the winter were replaced by missing values. The missing values were replaced (if possible) by estimated values calculated from highly correlated regression models using reliable data from other stations. Although not all gaps could be filled by such regression calculations the project database is considered to be highly reliable for the process analyses and the simulation exercise. The period of November 1994 to October 1995 has been used in this paper for analysing the hydrological dynamics of the test slope and for carrying out the simulation runs. 2.1.2. Climate Using hourly means of the weather data from K0, potential evapotranspiration (ETP) was calulated by means of the Penman–Monteith equation (ASCE, 1990), which was included into the HILLS model for this simulation study. According to Flu¨gel, 1978, 1993) precipitation and the actual evapotranspiration (ETA) are the major water balance input and output, controlling the generation of interflow and the hillslope drainage. ETA in turn depends on the plant transpiration activity which is expressed in HILLS as a function of the actual soil water storage. The precipitation in the hydrological year 1995 amounts to 1176 mm with a daily maximum of 58.2 mm in December 1994. From this annual total altogether 63% (746 mm) was received on the test slope
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during the winter half-year between November 1994 and April 1995. ETP was calculated at about 672 mm, of which the vegetation period between May and October 1995 accounts for 80% (537 mm). The distribution of rainfall and ETP for the hydrological year 1995 is given in Fig. 5. The partial water balance can be described as follows: (i) During November 1994 to April 1995 the slope received 746 mm of precipitation. ETP was calculated to about 135 mm, which gives a positive balance surplus of 611 mm (81.9%). (ii) In the summer (May to October 1995) altogether 430 mm of rainfall was recorded, which gives a negative balance of 107 mm if compared with the calculated ETP of 537 mm for this period. 2.1.3. Soil moisture, groundwater dynamics and runoff The interactive dynamics of the unsaturated and saturated zone can best be studied during the winter halfyear. This is shown together with the runoff of the Breidenbach Creek in Fig. 6 using 5-minute data for a 4-day time period in January, 1995. The dynamics of soil tension observed by the tensiometers at station K6 were also recorded in a similar way at the other five stations of the Kiefer-Hang although the actual tension values measured along the slope differed. Water levels recorded at K6 in the Breidenbach Creek were transferred into discharge given as L/s by a calibration curve, which was established from measured cross-profile flow velocities at different water levels. Fig. 6 shows the following elements of the hydrological dynamics of the test slope: (i) As the soil layer approaches saturation, even smaller rainfall events result in a rise of groundwater in the valley floor and runoff in the Breidenbach Creek. (ii) The soil moisture dynamics—expressed as soil tension in different depths—is similar during the three major storms. For the storm of January 28 it shows that: 쐌 During the storm infiltrating rainfall decreased the soil
Fig. 6. Rainfall, soil tensions, groundwater depths and runoff of the Breidenbach Creek at the stations K5 and K6 between 26–29 January 1995.
tension to near zero. This decrease occurred with a certain delay according to the installation depth of each instrument. At 90 cm depth some positive soil water pressures were recorded because of the groundwater head at the tensiometer cell. 쐌 At K6 infiltrating water percolated through the soil to the groundwater, bringing its level to about 40 cm below the surface of the valley floor. Groundwater is rising above the tensiometers and as a result these instruments are measuring positive soil water pressure instead of soil tension. 쐌 On the slope the percolating water reached the underlaying impermeable Devonian shale, generating a saturated zone and an interflow hydrograph. The latter was observed at station K5 when it reached the footslope zone. 쐌 After the end of the storm the soil started to drain again by gravity, providing the recession limb of the interflow hydrograph. (iii) Groundwater dynamics and runoff in the Breidenbach Creek reflect the rainfall and interflow inputs in the following way:
Fig. 5. Measured daily precipitation (P) and calculated potential evapotranspiration (ETP) during the hydrological year 1995.
쐌 Due to surface runoff from farm roads the runoff in the Breidenbach Creek responds almost instantly to the rainfall and slightly earlier than the rise of groundwater level in the valley floor at K6. 쐌 Comparing the rising limbs of the hydrographs from K6 and in the Breidenbach Creek, a steplike break can be seen during the rise. This break coincides with
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the delayed rise of the groundwater level at station K5 which was the result of the interflow hydrograph from the hillslope. 쐌 Further rise of the runoff hydrograph after this break obviously is caused by interflow from the test slope which is moving through the almost filled groundwater aquifer of the valley floor towards the Breidenbach Creek. 쐌 Comparing the similar dynamics of the hydrograph recessions of Breidenbach runoff and groundwater at K5 reveals that after the storm the runoff in the Breidenbach Creek is mainly controlled by interflow, draining the slopes of the catchment. This hydrological dynamics was frequently observed and is a typical response of the “dynamic total storage” as described by Flu¨gel (1993) in the valley floor and the footslope zone, which is jointly controlled by infiltration, groundwater recharge, interflow hydrographs and runoff contribution. 2.1.4. Hydrological dynamics of the Kiefer-Hang hillslope The results of the field survey, the climatic balance and the process studies discussed above reveal the hydrological dynamics of the Kiefer-Hang hillslope system as follows: (i) During the winter half-year the precipitation surplus (i.e. in 1995 about 611 mm) was infiltrating into the soil recharging the soil water storage. The soil infiltration capacity of such a grassland slope always exceeds the rainfall rate and consequently surface runoff on the slope was never observed. This is in line with results reported by Flu¨gel, 1978, 1993 from a test slope with similar vegetation, soil texture and topography near Heidelberg, Germany. (ii) At the beginning of the storm, infiltrating precipitation accumulates first in the soil layer and a wetted region grows from the soil surface. As this zone becomes wetter, infiltrating water moves more rapidly downwards, generating a saturated zone above the underlying impermeable Devonian shale. If macropores (e.g. from earth worms) are present water can also flow directly to the surface of the underlying bedrock. (iii) Due to the slope gradient interflow is generated within this saturated layer and moves laterally downslope along this layer towards the valley floor. This analysis is confirmed by the hydromorphic pseudogleyic character of the soils above the Devonian shale which was found all along the hillslope during drilling (Fig. 4). (iv) Interflow moves from the slope through the adjacent groundwater system, contributing to the runoff of the Breidenbach Creek. A variable time delay might be caused by an intermediate storage of the interflow as
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perched groundwater in the bedrock ditch shown at station K2 in Fig. 4. (v) During the summer half-year ETP is mostly higher than the rainfall input, as in 1995 when rainfall was 430 mm and ETP was calculated to about 537 mm. Precipitation which is not intercepted, infiltrates and is stored within the soil for a later use by evapotranspiration. Periods with groundwater recharge and interflow are rare and restricted to wet summerly conditions such as reported by Flu¨gel (1993). From this analysis it can be concluded that the hillslope drainage, the generation of interflow and its runoff contribution is mainly restricted to the hydrological winter half-year having a positive water balance. During the summer half-year the water balance is negative and the discharge of the Breidenbach Creek is established by occasional storm runoff and groundwater seepage during dry weather periods. 2.2. Modelling 2.2.1. Representing the hillslope The soil of the slope was differentiated into two layers, of which the lower one, the Devonian shale was regarded as being almost impermeable. The hillslope geometry (Fig. 1) can be described within HILLS by up to 20 nodes along the hillslope. At each node the depth of the surface soil and the slope gradient must be defined. HILLS also permits the identification of two checkpoints at which the dynamics of the perched watertable are reported. These points are essential for the calibration of the model, and the two tensiometers at stations K5 and K6 with recording groundwater level devices were used. 2.2.2. Parameterization A method of estimating hillslope parameters for the HILLS model was given by Hebbert and Smith (1990). The major physical parameters controlling the soil moisture dynamics within HILLS are: (i) soil volumetric saturated water content; (ii) saturated vertical hydraulic conductivity; (iii) an anisotropy factor, which is a quotient relating horizontal to vertical hydraulic conductivity; (iv) the pore-size distribution index; (v) the capillary fringe height; and (vi) the depth of the perched watertable at the bottom end of the hillslope. Disturbed and undisturbed samples collected at station K2 and K6 were analysed to parameterize the hillslope and the valley floor respectively. Basic soil physical parameters obtained from undisturbed samples from a soil pit in station K6 in the foot slope of the cross-section are listed in Table 1. For the description of the soil horizons the German soil classification has been used. The anisotropy will govern the value of Kh, and according to the soil
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Table 1 Soil physical parameters from tensiometer station K6 used to parameterize the HILLS model Soil horizon/depth Ah 0-15 cm Bv 16-30 cm Go 31-60 cm Gr > 60 cm
PV (%)
FC (%)
BD (g/cm3)
Ku (m/s)
54.1 54.1 43.0 41.7
49.3 46.8 39.3 35.6
1.12 1.10 1.46 1.50
1.55E-05 3.80E-05 3.46E-06 2.23E-07
Ah ⫽ humic A; Bv ⫽ weathered B; Go, Gr ⫽ groundwater influenced G; PV ⫽ total pore volume; BD ⫽ bulk dry density; Ku ⫽ saturated conductivity; FC ⫽ field capacity.
survey values between 1.0 and 1.2 have been chosen. The divergence angle ␦ used to describe a converging or diverging hillslope shape was set to zero, since the Kiefer-Hang hillslope is a relatively non-converging slope. Data from the tensiometer station K6 in the valley floor were used to validate the simulations of soil moisture and groundwater dynamics. 2.2.3. Groundwater dynamics The ability of the HILLS model to simulate the groundwater dynamics in the valley floor will be discussed for different time periods and considering the seasonal hydrological dynamics described above. All simulations are based on the measured 5-minute rainfall data and the corresponding groundwater dynamics, (note that an input interval of 5 min is not required by the HILLS model). The simulation results for a period of the winter half-year 1995 are shown in Fig. 7 and reveals: (i) Except for the storm in January 1995 the measured groundwater dynamics at tensiometer station K6 is reproduced reasonably well. Deviation between measured and simulated data can be explained by the simplification of the slope’s soil physical heterogeneity within the HILLS model. (ii) During heavy storms in January and February 1995 the HILLS model generates a water level which is
Fig. 7. Measured and simulated groundwater depths at station K6 between 2 January to 30 April 1995, based on 5-minute data intervals.
greater than measured at station K6. This discrepancy might be the result of several simplifications within HILLS: Inadequacy of the Dupuit–Forchheimer assumptions for the lower creek area, vertical heterogeneity of soil porosity, or inadequacy of the use of fixed water level for the downstream boundary condition, rather than a dynamic creek level. The simulations of the 10-month period from 1995 shown in Fig. 8 are also based on 5-minute continuous simulations, but for the presentation daily averages were used. This period comprised different dynamic hydrometeorological conditions, which in the HILLS model could only be described by constant model parameters. The following observations come from these results: (i) The agreement between the measured and simulated groundwater depth improved due to the averaging as positive and negative small time scale deviations were compensated. (ii) During the summer half-year the soils of the test slope lost water by evapotranspiration, and interflow ceased almost completely. In such dry weather conditions HILLS tends to overestimate the groundwater response corresponding to summer storms although the temporal groundwater dynamics was reproduced fairly well.
Fig. 8. Measured and simulation of groundwater depths at K6 between January and November 1995.
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(iii) Groundwater depths during periods with dominant dry weather conditions, i.e. in August end September 1995 were only poorly simulated by HILLS. The groundwater depth in the valley floor was underestimated probably because the model uses a fixed creek level for a lower boundary condition, and the Breidenbach Creek level is not updated in the model. It also seems that HILLS may not sufficiently account for the capillary rise of groundwater into the overlaying unsaturated zone from where it is used by evapotranspiration. 2.2.4. Runoff contribution by interflow As discussed before the contribution of interflow to runoff generation is of major importance in hilly catchments. The HILLS model simulates interflow seepage heights, given in mm, which are shown together with the measured discharge of the Breidenbach Creek in Fig. 9. Although the measuring units are different the following observations can be made from the comparison of the two time series: (i) Discharge dynamics of the Breidenbach Creek and interflow height simulated by the HILLS model from the test hillslope Kiefer-Hang have an almost identical dynamics, clearly demonstrating the importance of interflow for the runoff generation within this catchment. (ii) A time delay between measured runoff peaks and simulated interflow contribution might be caused from surface runoff generated on farm roads and outflow of interflow in the foothill zone. Both were observed in the winter periods meanwhile overland flow on the hillslope surface was not observed during the total measurement period.
3. Concluding discussion From the interflow study presented in this paper one may make the following conclusions:
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(i) Integrating hydrological process studies and continuous process simulations are a prerequisite to analysis of the hydrological hillslope and interflow dynamics. The concept of the dynamic total storage (Flu¨gel, 1993) could also be applied to the process study of the KieferHang hillslope. (ii) During periods with positive water recharge the HILLS model simulated the observed groundwater dynamics fairly reasonably. Deviations between measured and simulated time series can be explained by the simplified representation of the physiographic heterogeneity of the slope systems in the HILLS model, which cause some deviations from intermediate groundwater storages as observed at station K2 (Fig. 2). (iii) Groundwater level dynamics near the creek was poorly simulated during heavy winter storms and in summer periods with dominant dry weather conditions. Beside the use of a few constant parameters to describe a system which may have complex seasonal changes, the main reason for such a poor simulation is probably the absence of a time variable lower boundary based on the water level in Breidenbach Creek. In addition HILLS evapotranspiration simulation is very simple, and may be inadequate for capillary rise of water from the perched groundwater system for evapotranspiration during summer. (iv) Systems analyses using the measured hydrographs as well as the interflow simulations provided by the HILLS model confirm the importance of interflow for runoff generation on such catchments.
4. Future research direction Based on the discussions above, future research needs related to the problem of interflow should focus on the following subjects: (i) Additional integrated process and simulation studies on this hydrological component should be carried out in test catchments having different climate, geology and physiographic conditions. (ii) Physically based models such as HILLS should describe the complete sytem of the “dynamic total storage” (Flu¨gel, 1993) including the interaction between the aquifer in the valley floor and the adjacent stream.
Fig. 9. Measured runoff (Q) in the Breidenbach Creek and simulated interflow (I) contribution from the Kiefer-Hang hillslope over the period of January to April 1995.
(iii) Results from such integrated studies must be regionalised in distributed catchment models. The concept of Hydrological Response Units (HRUs) developed by Flu¨gel (1995) can be applied for such a regionalisation approach.
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References ASCE, 1990. Evapotranspiration and irrigation water requirements. ASCE Manuals and Reports on Engineering Practice, 70. American Society of Civil Engineers, New York, 323 pp. Anderson, M.G., Burt, T.P., 1990. Process Studies in Hillslope Hydrology. John Wiley and Sons, Chichester. Bronstert, A., 1994. Modellierung der Ablussbildung und der Bodenwasserdynamik von Ha¨ngen. IHW, Univ. Karlsruhe 46, 192. Chorley, R.J., 1978. The hillslope hydrological cycle. In: Kirkby, M.J. (Ed.) Hillslope Hydrology. John Wiley and Sons, Chichester, pp. 1–42. Flu¨gel, W.-A., 1978. Untersuchungen zum Problem des Interflow. Heidelberger Geographische Arbeiten 56, 275. Flu¨gel, W.-A., 1993. Hangentwa¨sserung durch Interflow und seine Regionalisierung Einzugsgebiet der Elsenz (Kraichgau). Berliner Geographische Arbeiten 78, 68–94. Flu¨gel, W.-A., 1995. Delineating hydrological response units (HRU’s) by GIS analysis for regional hydrological modelling using PRMS/MMS in the drainage basin of the River Bro¨l, Germany. Hydrological Processes 9, 423–436.
Freeze, R.A., 1978. Mathematical models of hillslope hydrology. In: Kirkby, M.J. (Ed.) Hillslope Hydrology. John Wiley and Sons, Chichester, pp. 177–226. Hebbert, R.H.B., Smith, R.E., 1990. Hillslope parameter estimation using the inverse procedure. Journal of Hydrology 119, 307–334. Hebbert, R.H.B., Smith, R.E., 1992. User manual for HILLS, Vers. 5.2, Numerical Hillslope Model. Unpublished Manual, US Department of Agriculture, Agricultural Research Service, Water Management Unit, Fort Collins, CO, USA. Hewlett, J.D., Troendle, C.A., 1975. Non-point and diffuse water sources: A variable source area problem. Paper presented on Watershed Management, Utah State University, Logan, ASCE, New York, pp. 21–46. Kirkby, M.J., 1978. Hillslope Hydrology. John Wiley and Sons, Chichester. ¨ ber Wald- und Wasserhaushalt im Ruhrgebiet. Kirwald, E., 1955. U RTV, Essen. Kool, J.B., van Genuchten, M.Th., 1991. HYDRUS Version 3.31. Unpublished Manual, US Salinity Laboratory, Agricultural Research Service, US Department of Agriculture, Riverside, California, USA.