The effect of broadleaved woodland on Chalk groundwater resources
[Supplementary Information]

John Roberts and Paul Rosier 

CEH-Wallingford, Wallingford, OX10 8BB, UK

[Paper for Quarterly Journal of Engineering Geology]

Supplementary Information 1: Sites and methods
Sites

Two Chalk sites were instrumented; a beech (Fagus sylvatica) woodland site in Black Wood, Micheldever, Hampshire (SU542432) and a grassland (perennial ryegrass, Lolium perenne) site, 7 km to the southwest at Bridgets Farm, Martyr Worthy, Winchester, Hampshire (SU519343). 

Black Wood
The soils at Black Wood are of the Andover series (Jarvis, 1973) and consist of about 50 - 60 cm of silty clay above Upper Chalk. At the interface with the clay, the Chalk is rubbly but grades within a few tens of centimetres to blocky Chalk. Observations of the vertical distribution of roots were made in a 4.5 m deep pit excavated in the forest and in soil cores. Tree roots were found to be profuse in the clay horizon but only enter a few centimetres into the rubbly Chalk where they are finely divided. The stand where measurements were made was almost pure beech, planted in 1950. A large part of this stand is on level ground, at an altitude of 100 m AOD, and is surrounded by a substantial extent of forest (approximately 2.5 km2), of similar structure, in all directions. The covering on the woodland floor is patchy, with about 50% of the area having only leaf litter and the remainder having blue bells (Hyacinthoides non-scripta) in the spring or occasional patches of dog’s mercury (Mercurialis perennis). 
Bridgets Farm
At the time of the studies reported here, Bridgets Farm was an experimental (arable and dairy) farm tenanted by the Agricultural and Development Advisory Service (ADAS). The field chosen to locate the soil moisture measurements is known on the farm as ‘Delaware’ and had been in pasture for several years. It is approximately 11 ha in area and has an altitudinal range of 90 to 105 m AOD. The soils are again of the Andover series (Jarvis, 1973; Moffat, 1985), with the silty-clay layer above the Upper Chalk being somewhat shallower (30 - 40 cm deep) than at Black Wood. The uppermost layers of Chalk consist predominantly of small Chalk nodules. This fragmented Chalk extends to a depth of around 1.5 m where the Chalk then becomes consolidated. Grass roots extend only a few centimetres into the loose Chalk.

Measurements

Rainfall
A 28.5 m high instrument tower was installed in the forest at Black Wood which extended about 6 m above the tallest surrounding trees. An automatic weather station (AWS) was mounted on the tower and there was also a rain gauge funnel (308 cm2) connected by a polythene tube to a 0.5 mm tip-1 rain gauge located at the base of the tower. Data from the AWS and rain gauge were recorded hourly onto a data logger (Model CR10, Campbell Scientific Ltd., Shepshed, UK). To augment the above-canopy rainfall measurements, a 0.2 mm tip-1 tipping bucket rain gauge was installed in a large clearing some 500 m away from the tower site. Data from this rain gauge were recorded hourly on a miniature solid state data logger (Model Tiny Tag Plus, Gemini Dataloggers, Chichester, UK). An AWS has been installed at Bridgets Farm for several years and hourly meteorological data including rainfall were available. The AWS was located in an enclosure within a hedgerow some 500 m away from the soil moisture measurement plot. To augment the farm AWS rainfall measurements, a 0.5 mm tip-1 tipping bucket rain gauge was installed within the soil moisture measurement plot. Data from this rain gauge were also recorded hourly on a miniature solid state data logger.

In addition to the rainfall data available from the two experimental sites, there was a daily rainfall record available from a rain gauge located between the two sites at East Stratton (SU 543396). This is a daily rainfall measurement station reporting to the Meteorological Office and the Environment Agency. 

Soil water content
Measurements were made of changes in the soil water content of deep profiles with a neutron probe (Model IHII, Didcot Instrument Company, Didcot, Abingdon, UK). The same neutron probe was used on all occasions and readings were always made at Black Wood and Bridgets Farm on the same day. Neutron probe access tubes consisting of thin-walled 45 mm id aluminium tubing were inserted as a tight fit into vertical holes that had been drilled using a hydraulic rotary drilling rig with air-flush. In each neutron probe access tube, readings were taken every 10 cm from the soil surface down to 120 cm and then every 30 cm to the lowest reading depth in the tube. Measurements were usually made at weekly intervals. At Black Wood, data were collected from five access tubes installed deep enough to allow a minimum reading depth of 7.8 m in each tube. At Bridgets Farm, there were six, 9 m deep neutron probe access tubes installed in a 7 x 7 m fenced enclosure at the southern edge of the Delaware field. During the measurement period, the field was periodically grazed by dairy cattle. Whenever possible the height of the grass in the enclosure was maintained at the same height as the grass outside by the use of a battery-powered strimmer. In addition, because Delaware field had occasional applications of slurry which was not applied to the enclosure, grass fertiliser was broadcast evenly, at the appropriate application rate, after the grass had been cut.

Soil water potential
A principal need in this work was to measure soil water potentials in deep soil profiles.  A number of studies of Chalk soil water potential indicated that the potentials to be measured were likely to be well beyond the range of conventional water-filled tensiometers. The use was made of  ‘Equitensiometers’ (Model EQ2, Delta-T Devices, Burwell, Cambs., UK) to measure soil matric potential (over the range of 0 - 1 MPa).   

In January 1999, profiles of 8 Equitensiometers were installed at Black Wood and Bridgets Farm. The profile consists of 80 mm diameter vertical holes drilled at 1 m depth intervals from 2 m down to 9 m. The holes were drilled with the same drilling rig that was used to drill the holes for the neutron probe access tubes. Each hole had all but the bottom metre lined with a tight-fitting plastic drainage pipe and a Chalk slurry was poured down the hole to a depth of approximately 40 to 50 cm from the bottom. The plastic extension (1 m) of the Equitensiometer, had been fitted inside a length of aluminium tube through which its cable and connecting plug had been threaded. The length of aluminium tube attached to each Equitensiometer allowed the unit to be lowered down the hole and pushed into the slurry while a short length of aluminium tube remained above ground level. Spacers were used to centrally locate the aluminium extension tube of the Equitensiometer centrally within the plastic liner of the drilled hole. The open tops of the aluminium tube and plastic liner were then sealed with silicone compound. Additional Equitensiometers were installed at both sites, at depths of 0.8 and 1.6 m in May 1999, and at 0.4 and 1.2 m in March 2000. The four units, installed above 2 m at each site, were placed in inclined holes, which were only slightly greater in diameter than the Equitensiometer. The Equitensiometers were installed directly into contact with the soil at the bottom of the hole, without any Chalk slurry being used.

The Equitensiometers at each site were read manually with a meter at weekly intervals until November 1999 when a 12 channel CR10 data logger (Campbell Scientific Ltd., Shepshed, Leics., UK) was installed at each site to record data from the units hourly. The Equitensiometers at 8 and 9 m at each site continued to be measured weekly with the meter. 

Evaporation estimated from measurements of the energy balance
The surface energy balance can be stated as:
 
Rn + G + H + lE + S = 0		(1)

where Rn is the net radiation, G is the soil heat flux, H is the sensible heat flux, lE is the latent heat flux (evaporation) and S represents heat storage in the biomass and within the canopy air space. Changes in biomass storage on a diurnal basis are likely to be significant at Black Wood and would be required should precise hourly estimates of latent heat flux need to be estimated. For present purposes any changes in heat storage remain as a residual along with estimates of the latent heat flux. Over 24 hour periods, however, negative and positive changes in heat storage in the biomass and air space (i.e. S) will tend to cancel out and the energy balance equation then reduces to a simpler form,

          Rn + G + H + lE = 0			(2)

For clarification it should be noted that evaporation comprises transpiration, i.e. water taken up from the soil and lost from the foliage during dry weather, and/or evaporation of surface water from leaves wetted by rain or dew. In winter evaporation from the forest in dry conditions is from the litter layer drying out.

Energy balance measurements at Black Wood and Bridgets Farm
Net radiation was measured with 2, south-facing net radiometers (Model Q-7, REBS, Seattle, Washington, USA) mounted at the top of the instrument tower. Soil heat flux was measured with 2 soil heat flux plates (Model HFP01, Hukseflux Thermal Sensors, Delft, The Netherlands) installed about 50 mm below the soil surface close to the tower. The sensible heat flux is calculated from measurements of the fluctuations in the vertical wind speed and the associated fluctuations in air temperature. A ‘Solent’ (Gill Instruments, Lymington, UK) Model A1012R three-dimensional sonic anemometer was installed on the top of the instrument tower 7.3 m above the main canopy. This instrument measured the horizontal and the vertical components of the wind speed, the speed of sound and the air temperature with a fine thermocouple (Model EMQSS M025E-300, Omega Engineering Ltd., Manchester, UK.). The derivation of sensible heat fluxes, from the instrument, assumes the principle of eddy covariance in which upward and downward moving eddies transport heat to or from the vegetation surface. The instrument measured the net vertical flux of heat. Evaporation was estimated using a combination of information from eddy covariance measurements, i.e. the ‘Solent’ anemometer, and the energy balance (Equations 1 and 2). In subsequent text this approach is referred to as the Solent/energy balance. The sampling rate was 5Hz and the data stored on a Model CR21X Micrologger (Campbell Scientific Ltd., Shepshed, Leics. UK). Data were retrieved from the logger on the same day as other data were retrieved from Black Wood and Bridgets Farm.

An identical three dimensional sonic anemometer, thermocouple, net radiation, soil heat flux and logging system to that at Black Wood was installed in the middle of Delaware field at Bridgets Farm. The anemometer was mounted on a pole at a height of 2.5 m in a 5 x 5 m enclosure surrounded by electric fences. The net radiation and the soil heat flux were also measured in the same enclosure. 

Drainage estimated from the water balance equation
Assuming that there is no surface runoff, for any period when all values are available, the components of the water balance (in mm), rainfall (P), evaporation (E) and changes in soil moisture (DS) are related to drainage (D) by the equation;

			P - E - DS - D = 0			 (3)		

Values of rainfall and soil moisture were available from both sites for the whole of the project. For the purposes of calculating a water balance, the daily rainfall record from East Stratton was used. Directly-measured evaporation data were only available for the period when the energy balance information was available for Black Wood and Bridgets Farm. The evaporation measured at Bridgets Farm showed a very close agreement with evaporation calculated using the Penman potential evaporation formula (Penman, 1948). Therefore the evaporation from Bridgets Farm could be extended by using the Penman formula. When data were not available from the energy balance system used at Black Wood in winter, an estimate of 0.25 times the Penman estimate was used. The value of 0.25 was derived from comparisons of Penman estimates with energy balance measurements made in leafless periods in the Spring of 1999 and the autumn/winter of 1999 and following spring of 2000. The shortest time interval over which drainage could be calculated was weekly because this was the interval at which the soil water content was measured.  In practice the main interest centred on a comparison between Black Wood and Bridgets Farm on either a seasonal or an annual basis.

Jarvis, M. G. 1973. Soils of the Abingdon and Wantage District. Memoirs of the Soil Survey of Great Britain, England & Wales, Harpenden, UK.

Moffat, A. J. 1985. Soils of Bridget’s Experimental Husbandry Farm, Martyr Worthy, Winchester. Soil Survey of England & Wales, Rothamsted Experimental Station, Harpenden, Herts.

Supplementary Information 2: Results from other broadleaved woodland and grass comparisons.
Figures S1, S2 and S3 show comparisons of soil water changes below broadleaved woodland and grass in three separate studies. Figure S4 shows a comparison of evaporation from broadleaved woodland and grass.

Figure S1. Seasonal changes in soil water content below oak (——) and grassland (▬ ▬ ▬) at Straits Inclosure relative to the soil water content in the Spring of 1993 (after Bending and Moffat, 1997).

Figure S2. Seasonal changes in soil water content below oak (——) and grassland (▬ ▬ ▬) at Clipstone relative to the soil water content in the Spring of 1998 (after Calder et al., 2000).

Figure S3.  Seasonal changes in soil water content below oak (——) and grassland (▬ ▬ ▬) at Well House Farm relative to the soil water content in the Spring of 1998 (after Finch 2000).

Figure S4. Cumulative evaporation from woodland (——) and grassland (▬ ▬ ▬) in the Kingston Brook catchment, Leicestershire in a). 1969 and b). 1970. Data after McGowan and Williams (1980). 

Figure S1

Figure S2

Figure S3

Figure S4