HYDROLOGICAL PROCESSES, VOL. 10, 1081-1089 (1996)

MEASURING AND MODELLING OF SOIL WATER DYNAMICS AND RUNOFF GENERATION IN AN

AGRICULTURAL LOESSIAL HILLSLOPE

COEN J. RITSEMA, JANNES STOLTE, KLAAS OOSTINDIE AND ERIK VAN DEN ELSEN DLO Winand Staring Centre for Integrated Land, Soil and Water Research (SC-DLO), PO Box 125,

6700 AC Wageningen, The Netherlands

AND

PAUL M. VAN DIJK Laboratory of Physical Geography and Soil Science, University of Amsterdam, Nieuwe Prinsengracht 130,

1018 VZ Amsterdam, The Netherlands

ABSTRACT Surface runoff may be generated when the rainfall intensity exceeds the infiltration capacity, or when the soil profile is saturated with water. Indications exist that both types of overland flow may occur in hilly agricultural loess regions. Here, for a loessial hillslope under maize in the southern part of The Netherlands, it was shown, with pressure head and runoff measurements, that Hortonian overland flow occurs during typical summer rain events. Surface runoff was initiated after saturation of the top 5-10cm of the soil. Deeper in the soil, unsaturated conditions prevailed while runoff took place. Peak runoff discharges at the outlet of the subcatchment occurred a few minutes after peak rainfall intensities were measured. It appeared that SWMS_2D, a two-dimensional water flow model, was capable in simulating observed pressure head changes and runoff. Simulated potential runoff for the transect studied was higher by a mag- nitude of three than the measured areal average. This indicates effects of surface ponding, and the probable location of this particular transect in a region with high runoff production.

KEY WORDS surface runoff; overland flow; soil erosion; pressure heads; two-dimensional water flow model

INTRODUCTION

Water erosion is a widespread phenomenon causing important losses of water, soil and nutrients in elevated regions. Hilly, agricultural loess areas are particularly susceptible to erosion (Richter, 1978; De Ploey, 1983, 1986; Boardman, 1990; Evans, 1990; Kwaad, 1991; Mathier and Roy, 1993).

In general, two types of overland flow can be distinguished, namely Hortonian overland flow which occurs when the rainfall intensity exceeds the infiltration capacity (Horton, 1933), and saturation overland flow which occurs when the whole soil is saturated with water thus preventing any further infiltration (Dunne et al., 1975; Dunne, 1978).

Hortonian overland flow in loess soils is often associated with surface seals or surface crusts. Numerous studies have indicated that bare or partly covered agricultural loess soils are susceptible to crust formation (Poesen and Govers, 1986; Mualem and Assouline, 1989; Luk and Cai, 1990; Mualem et al., 1990; De Roo and Riezenbos, 1992). Crust formation goes hand in hand with a decrease in soil hydraulic conductivity of the top layer, which, depending on rainfall intensities and duration, can be more or less severe (McIntyre, 1958; Hillel and Gardner, 1969, 1970; Baumhardt el al., 1990; Romkens ef at., 1990; Gimknez et al., 1992). A decrease in soil hydraulic conductivity of the top layer generally promotes the occurrence of Hortonian overland flow.

CCC 0885-6087/96/08/108 1/09 0 1996 by John Wiley & Sons, Ltd.

Received I0 April 1995 Accepted 24 August 1995

1082 C. J. RITSEMA ET AL.

Saturation overland flow may occur when an impermeable bedrock or dense clay layer underlies the soil profile. Examples of such conditions have been reported by Hewlett (1974), Dunne (1978) and O'Loughlin (1981, 1986). Besides the inducement of saturation overland flow by impermeable bedrocks or dense clay layers, a substantial subsurface flow over these layers may be generated as well (Weyman, 1973; Ahuja and Ross, 1983; Selim, 1987; Parlange et al., 1989; Koide and Wheater, 1992). In cases where the break in vertical hydraulic conductivity is less severe, for instance in the situation with a more permeable over a less permeable layer, subsurface flow may be induced as well, as indicated by Michiels et al. (1989).

Recently, Kwaad (1991) concluded, from extensive surface runoff and soil loss measurements on a fallow loess soil in the Netherlands, that Hortonian overland flow is likely to occur during summer rain events, and saturation overland flow during winter rain events. However, this was not verified by simultaneous pressure head or soil water content measurements. Therefore, the aims of this study were (i) to quantify the main water flow pattern through a loessial hillslope during a natural rain event, (ii) to simulate observed water flow with a two-dimensional water flow model, and compare measured and simulated pressure heads and surface runoff, and (iii) to evaluate the main process mechanism leading to surface runoff.

MATERIALS AND METHODS

Site and soil A hillslope under maize in a small subcatchment (around 4 ha) near Ransdaal in the southern part of

The Netherlands was selected to monitor water flow patterns through an instrumented transect. The total length of the selected transect was slightly more than 100 m. Three soil types could be distinguished

I I 1 I I I

I" 12

10

8

6

4

2

n " 0 20 40 60 80 100 120

Distance (m)

Figure 1. Schematic side-view of the studied hillslope with distinguished soil horizons (a) and the finite element grid (b) as applied in the two-dimensional simulations

LISEM 7: SOIL WATER DYNAMICS AND RUNOFF GENERATION 1083

Table I. Mualem-Van Genuchten parameters for the five different soil horizons as used in the model simulations

Horizon thr ths alfa n 1 Kk

Plough layer 0.01 0.409 0.0125 1.124 0.5 0.100 Loess colluvium 0.01 0.391 0.0042 1.318 0.5 0.414 Loess 0.01 0.409 0.0114 1.166 0.5 3.033 Weathered chalk 0.01 0.405 0.0066 1.247 0-5 2.640 Chalk 0.01 0.494 0.0165 1.444 0.5 11.546

in this transect: a mesic Typic Udorthent (loess in sizu) at the top, a mesic Lithic Udorthent in the middle and a mesic Typic Udorthent (loess colluvial) at the bottom part. A schematic side-view of the particular transect with the distinguished soil horizons is shown in Figure la.

Each horizon was sampled twice to measure the soil hydraulic properties. Water retention and the unsat- urated hydraulic conductivity functions were measured using the evaporation method of Wind (Wind, 1968; Boels et al., 1978), and the saturated conductivity with the constant head method as described by Klute (1986). Measured functions were curve fitted using the RETC programme developed by Van Genuchten et ai. (1991), in order to obtain the Mualem-Van Genuchten parameters (Mualem, 1976; Van Genuchten, 1980). The soil physical properties used in this study are summarized in Table I.

Raingauge 4

Connection for computer --.)

180 I

Pressure head box containing: - Dressure transducers - data logger -flush equipment

I

Figure 2. Installation method for the stand alone pressure head measurement device, and the connected tensiometers and rainfall gauge

1084 C. J. RITSEMA ET AL.

l P - .- 0

50 - -

1509 + 100 - - If

+18:19

I -

Field set-up The selected transect was instrumented with three stand alone devices as developed by Van den Elsen and

Bakker (1992) to measure hydraulic heads. The devices were installed at the top, middle and bottom part of the hillslope. On each device, ten tensiometers were connected, placed at 2.5 (twice), 5 (twice), 10,20,40, 60, 80 and 120 cm depth, respectively (Figure 2). The measurement frequency was once every hour during dry periods and once every five minutes during and just after rain events. Switching from the low to high frequency measurement scheme was regulated through the first signal of the connected rainfall gauge.

Runoff at the bottom of the subcatchment was measured in a 1.15-m wide and 0.62-m high H-flume (Bos, 1989). By using a pressure transducer, water discharges exceeding 0.5 1 s-l could be monitored. In case the water level in the H-flume exceeded 5 cm, water samples were collected automatically once very five minutes in order to determine sediment budgets.

Modelling Observed pressure head changes during rain events in the selected hillslope were simulated using a

slightly adapted version of the SWMS-2D model (Simunek et al., 1992). SWMS-2D is a numerical finite element model for calculating two-dimensional water flow in variably saturated porous media. Water flow is described by a modified form of the Richard's equation, in which the role of the air phase in the water transport is assumed to be negligible (Simunek et al., 1992). The governing flow and transport equations are solved numerically using Galerkin-type linear element schemes. SWMS-2D can handle flow regions delineated by irregular boundaries.

Two-dimensional infiltration and water flow through the studied hillslope was simulated for the total duration of several rain events in the period 4-6 July 1992. The finite element grid used is shown in Figure lb. The distinguished elements in the top 30cm of the transect are not shown in Figure lb. In total, 121 nodes and 100 quadrilaterals were distinguished. SWMS-2D automatically subdivides the quadrilaterals into triangles, which are then treated as subelements.

Measured rainfall was used as the upper boundary condition. Evaporation was neglected, as it is of minor importance during high-intensity single rain events of short duration. The upslope vertical boundary and the bottom boundary (fixed at 180cm depth) were assumed to be no-flux boundaries. The lower slope vertical boundary was assumed to be a seepage face.

The soil horizons distinguished were assumed to be homogeneous, non-hysteretic and isotropic. For each soil horizon one set of soil physical properties was used in the model simulations, as given in Table I. To account for macropore flow through worm holes, modified Van Genuchten equations were used (Vogel and Cislerova, 1988) to add extra flexibility to the description of the hydraulic properties near saturation.

40 z

30

20

10

RESULTS AND DISCUSSION

Measured rain amount and intensity During the period 4-6 July 1992, a total of 32mm of rain was recorded (Figure 3). A few single rain

' 0 4 5 6 7

July 1992

Figure 3. Measured (cumulative) rainfall and rainfall intensity on the studied hillslope, 4-6 July 1992

200' '

LISEM 7: SOIL WATER DYNAMICS AND RUNOFF GENERATION

0 -

20 - E Y c 2 40 - 2 P s 5 60 - n 5 8 n

- .-

0

Time (h) 80 - O------ 0 15.00 0-0 17.00 A---A 18.20

100 -

120 * ‘ I I I I -250 -200 -150 -100 -50 0 -250

1085

- b

- L

-

-

-

i I -

I I I - J -200 -150 -100 -50 0 -250 -200 -150 -100 -50 0

I I I 5 6 7

July 1992

Figure 4. Measured pressure heads versus time at 5 , 10, 20 80 and 120cm depth at the bottom section of the studied hillslope

showers occurred in this period. On 4 July the period started with a low-intensity, 7-mm rain shower, which caused no overland flow. A second, 5-mm rain event was recorded later that day with a peak intensity of almost 70 mm h-’ . This shower caused runoff. At the end of 5 July, several rain showers were recorded close together (see Figure 3). Two of these showers produced around 6 and 8mm of rain, both resulting in sig- nificant overland flow. Peak rainfall intensities for these two showers were more than 100 mm h-’ and 150 mm h-’ , respectively.

Measured versus simulated pressure heads During the period 4-6 July 1992, pressure heads were recorded automatically at the top, middle and

1086 C. J. RITSEMA ET AL.

-250 ’ I 5 6 7

July 1992

Figure 6. Simulated versus observed pressure heads at 5,20 and 40cm depth for the top section of the hillslope during 5 and 6 July 1992

bottom part of the hillslope. In Figure 4 a selection of measured pressure heads for the bottom part of the hillslope are shown. It is noticeable that the initial pressure heads at the start of this period were low (less than -1OOcm) for all depths, indicating that the soil is relatively dry. A rapid response on the tensiometer at 5 cm depth and a retarded reaction on the tensiometer placed at 10 cm depth were observed on the first and second rain event on 4 July. No response was monitored on one of the other tensiometers. During the two important rain events on 5 July, pressure heads increased rapidly up to values around zero, indicating that at least the top 5cm of the soil was completely saturated with water (see also Figure 5). Once these showers had ended, a further downward migration of water into the soil profile could be detected through the pressure head readings at depths of 80 and 120 cm (see Figure 4). The slight but rapid increase in pres- sure head at 120cm depth during the second rain shower on 5 July is presumably caused by water flow through worm holes. During the other rain events this process was not observed, suggesting that biopore flow is activated only in cases where the top layer is fully saturated with water. On 6 July, pressure heads at all depths decreased slowly due to the redistribution of water and possible evaporation effects (Figure 4).

In the model simulation, measured pressure heads just at the start of 4 July were used as input to initialize the studied transect. Simulated pressure heads for the tensiometers installed at 5, 20 and 40 cm depth at the top of the hillslope are shown in Figure 6, together with the observed ones. It can be concluded that simu- lated and observed pressure heads agree quite well. In Figure 7 simulated pressure heads for all tensiometers are shown versus observed ones for the entire measurement period. From this graph it may also be con- cluded that the model is capable of simulating the observed processes reasonably well.

Measured versus simulated overland flow During the period 4-6 July 1992, overland flow occurred three times, namely during the second

shower on 4 July and during the two high-intensity showers at the end of 5 July (Figure 8). Around

Observed pressure heads (cm) -350 -300 -250 -200 -150 -100 -50 0 0

-50

$

-150:

-200 $ E

-100

&

f Y)

-250 2 n - 3

-300

-350 -

Figure 7. Simulated versus measured pressure heads for all tensiometers, 4-6 July 1992

LISEM 7: SOIL WATER DYNAMICS AND RUNOFF GENERATION

rn W

r

0

- 5 - -

151l-c

f -

4- 1822 10 -

J

15

1087

25 I ?

20

15

10

- 5

0

- Rainfall Runoff

Time (h)

Figure 9. Measured rainfall intensity and runoff discharge for the second rain event on 5 July. Note the slight retardation between the moment of highest rain intensity and the moment of highest runoff discharge.

22m’ of overland flow were measured at the outlet of the subcatchment, the main part of which was produced during the last rain event (approximately l2m’). Peak discharges were around 5, 10 and 13 1 s-’, respectively (Figure 8). In general, peak discharges at the end of the subcatchment were recorded about 2-3 minutes later than the registration of the highest rain intensities (compare Figures 3 and 8), indicating that generated runoff travelled rapidly towards the installed H-flume. Figure 9 shows meas- ured rainfall intensities and recorded runoff for the second rain event on 5 July in more detail. the delay of a few minutes between the moment of highest rainfall intensity and the moment of highest discharge at the outlet of the subcatchment is clearly demonstrated here.

Simulations with the SWMS-2D model yielded values for potential runoff for the studied transect. Potential runoff equalled rainfall minus infiltration. In Figure 10 calculated potential runoff is shown

Measured - 20 1 - Simulated

E 0 -

3 5

1

5 6 7 0

4 July 1992

B f

1.0 2. L 2

0.5

0.0

Figure 10. Simulated potential runoff (rainfall minus infiltration) for the selected hillslope versus the actual measured runoff at the outlet of the subcatchment

1088 C. J. RITSEMA ET AL.

together with measured runoff at the outlet of the subcatchment. Measured runoff for the entire subcatch- ment appeared to be around 22 m3. If runoff was generated uniformly over the entire subcatchment (around 40 000 m2), then each square metre would have generated roughly 0-5 mm of runoff. For the selected hill- slope the calculated potential runoff appeared to be around three times higher than this (approximately 1.5 mm m-2). This difference might be caused by the fact that (i) significant amounts of water probably ponded on the soil surface of the hillslope (see for instance De Roo et al., 1994), causing the actual gener- ated runoff to be lower than the 1.5 mm of calculated potential runoff, and (ii) runoff generation was prob- ably non-uniformly distributed over the subcatchment. The selected hillslope was one of the steepest in the subcatchment and located relatively close to the outlet, and thus likely to contribute more than the areal average of 0.5 mm to the observed runoff.

CONCLUSIONS

For a loessial hillslope under maize it was demonstrated that Hortonian overland flow occurred during a series of rain events in the summer of 1992. Before overland flow started, 5-lOcm of the soil became saturated with water. Deeper in the profile unsaturated conditions still prevailed. During single rain events peak runoff discharges were measured 2-3 minutes after monitoring of peak rainfall intensities. It appeared that the two-dimensional SWMS-2D water flow model was capable of simulating measured pressure heads and surface runoff. Simulated potential runoff from the transect appeared to be three times higher than the measured areal average, indicating the effects of surface ponding and possible spatial variability in runoff production.

ACKNOWLEDGEMENTS

This study has been carried out within the context of the project ‘Erosienormeringsonderzoek Zuid- Limburg’ initiated in 1991, and financed by the province of Limburg, the Waterboard ‘Roer en Overmaas’, the Ministry of Agriculture and 14 municipalities in the loess region of The Netherlands. Numerous col- leagues participated in instrumenting the four hillslopes with stand alone pressure head devices and H-flumes, in maintenance of field equipment and in data collection, of whom Edgar Vos, Hans Brons- wijk, Pim Hamminga, Gerard Veerman, Rik van den Bosch, Louis Dekker, Ed de Water, Eugene Sabajo and Martijn van der Zijp are particularly thanked. Furthermore, the assistance of the students Jeroen van Logtenstein, Marco Sprong and Gert-Jan Verhoeff was highly appreciated. Wim Leenders is thanked for mapping and soil classifications.

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