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The design of a portable rainfall simulator infiltrometer
Journal of Hydrology, 41 (1979) 143--147
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Technical Note [4] THE DESIGN OF A PORTABLE R A I N F A L L SI1VULATOR INFILTROMETER
ALAN S. T R I C K E R
Department of Geography, University of Dundee, Dundee (Great Britain) (Received September 20, 1978; revised and accepted October 11, 1978)
ABSTRACT Tricker, A.S., 1979. The design of a portable rainfall simulator infiltrometer. J. Hydrol., 41 : 143--147. The construction of a hand-portable rainfall simulator infiltrometer in described.
INTRODUCTION
Field measurements of infiltration m a y be attempted using either cylinder or rainfall simulator infiltrometers. The chief advantage of rainfall simulator infiltrometers is that they do give some indication of the rate of infiltration of rainfall as opposed to the ponded-water conditions of cylinders. Disadvantages of rainfall simulator infiltrometers rest with their cost and transportability. Many of these instruments are large, and usually require a vehicle to carry them and their water supply. However, Adams et al. (1957), McQueen (1963) and Selby (1970) have all produced hand-portable rainfall simulating infiltrometers which produce rainfall by the drip-screen method. In the design of Adams et al. drops were produced on the ends of glass capillary tubes. This was considered t o o delicate for field use, and so the drop production m e t h o d of McQueen was tested. A capillary hole (0.8 mm) was drilled through a piece of Perspex ® , 1.0 cm thick. A short length of 24 gauge (0.58 mm) wire was suspended through it b y bending each end of the wire. A small counter-sink drilled at the base of the hole prevented drops flowing across the Perspex surface, and together with the pin, provided an area for each drop to attach and grow. A 2.0 cm diameter disc of Perspex with a single capillary hole drilled through and m o u n t e d in a 5 cm length of polythene tube formed the individual "dropper-unit" from which characteristics of the simulated rainfall were determined. The diameter of raindrops produced was 2.83 ram. However, the diameter was found to be sensitive to any impurities in the water which may reduce surface tension effects, and thus reduce the
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growth period of drops. Hence the cleanliness of the dropper unit was of paramount importance. Measurements of the fall velocity drops was made using a photographic technique described by Bryan (1967). It was found t h a t the average velocity attained was 5.1 m/s with a fall of 1.5 m. Drops of 2.83 mm diameter, falling at this speed would have the same energy per unit of precipitation as raindrops of 1.4 m m diameter. From the data of Laws and Parsons (1943) this would be the average drop size of rainfall of 2.5 m m / h intensity. INFILTROMETER DESIGN
A compact, portable rainfall simulator infiltrometer was constructed on the basis of a 1.5 m fall height {Fig. 1). The instrument is best described within the framework of its component parts: (a) Drop-producing chamber (Fig. 2). The top and base of this chamber were constructed of 1 cm thick Perspex plate. The Perspex cylinder separating them had an external diameter of 15 cm, and was 12.5 cm deep. The base was cemented into the cylinder, although the top was detachable and had an air hole drilled through the handle. In the base 91 holes were drilled and countersunk in the fasion already described for the "single dropper unit". A tube of 0.8 cm diameter at the side of the cylinder, next to the base allowed water
Fig. 1. The portable rainfall simulator infiltrometer.
145
Fig. 2. The reservoir unit of the rainfall simulator infiltrometer, showing water drops forming at the base of counter-sunk capillary tubes. to e n ter the chamber. A measuring scale was attached to the side of the cylinder to enable changes in the head of water over the capillary tubes to be measured. (b) Float chamber. This was utilised to maintain a constant water head over the capillary tubes. It was constructed o f a 10 cm length of Perspex ® tube, 5 cm in internal diameter, with walls of 1.2 cm thick. The base of the chamber was milled o u t o f Delrin ® (a non-absorbent n y l o n derivative), t hrough which two pipes passed. One pipe was a direct link with the drop producing cylinder, while the o t h e r linked the float chamber and the reservoir. Within the latter tube a stainless steel needle valve was em be dd ed, which was also c o n n e c t e d to a float of ex p an d ed p o l y s t y r e n e (Fig. 3). T he needle valve prevented furt her flow o f water to the d r o p producing chamber once a pre-determined level had been attained. In this m anner a constant head of water could be maintained above the capillary tubes and thus a uni form intensity of rainfall produced. A threaded rod attached to the t op of the float chamber allowed it to be raised or lowered, and in this way, a range o f rainfall intensities could be produced. (c) Reservoir. This consisted of a 5 1 bot t l e m o u n t e d 8 cm above the dropproducing chamber and calibrated in centimetres head of water per unit area o f the plot (15 cm diameter). (d) Wind shield and support. T he Perspex wind shield was 1.35 m long,
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Fig. 3. The float-chamber control mechanism of the rainfall simulator infiltrometer. and had the same internal diameter as the drop-producing chamber. A PVC ring attached to the base prevented splintering, and a similar ring at the top allowed the drop-producing unit to be slotted into place. The whole instrument was supported by tripod legs braized on to a steelring around the wind shield. A small bubble-level attached to the top of one of the legs allowed the instrument to be erected vertically. (e) Base and runoff collector. The basal cylinder had an internal diameter of 15.0 cm. Angled side-walls prevented any water loss by splash. R u n o f f was removed from the plot to calibrated accumulator tubes through flexible polythene tubes using a small hand-pump. FIELD USE A description of the field use of the infiltrometer follows readily from the description of its manufacture. In the initial tests carried out with the infiltrometer, the plot border was driven 5 cm into the soil in order to allow comparison with infiltration capacities measured by cylinder infiltrometers that were installed in the same manner (Tricker, 1977). At other times, to avoid disturbance of the ground surface, the plot border could be sealed to the ground surface using clay (Selby, 1970). After the plot border and r u n o f f pipe are installed the instrument is erected vertically over the plot, using the bubble-level attached, and the height of the rain producing plate above
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the ground is adjusted using the tripod legs. A gap of 2.0 cm is left between the top rim of the plot border and the b o t t o m o f the windshield. A galvanished metal tray is inserted in this gap in order to intercept the rainfall until the level in the drop-producing chamber is reached (1--2 min) which will give the required rainfall intensity. At this time the operator slides the galvanised tray clear of the plot, notes the levels in the reservoir and drop-producing chamber, and starts the hand-operated r u n o f f pump. Readings of water levels in the reservoir, chamber and r u n o f f accumulation tubes are taken every five minutes. For each time interval the infiltration rate is calculated from: f=AP-AR-+AC where f = mass infiltration (cm/h); AP = change in reservoir level (cm/h); AR = change in r u n o f f a m o u n t (cm/h); and AC = change in chamber level (cm/h). CONCLUSIONS
The rainfall simulator infiltrometer described can be carried, set up, and operated conveniently by one man and offers a fast, economical means of measuring infiltration capacity. It requires only 1.5--2 man hours per measurement, and thus 3--4 measurements m a y be accomplished in a day. ACKNOWLEDGEMENTS
This work was accomplished while the author held a N.E.R.C. Research Studentship at the University of Sheffield. The instrument was constructed by Mr. C. Fletcher of the Geography Department Workshop.
REFERENCES Adams, J.E., Kirkham, D. and Nielsen, A., 1957. A portable rainfall simulating infiltrometer and physical measurements of soil in place. Soil Sci. Soc. Am., Proc., 21: 473--477. Bryan, R., 1967. The erodibility of some Peak District soils. Ph.D. Thesis, University of Sheffield, Sheffield (unpublished). Laws, J.O. and Parsons, D.A., 1943. The relation of raindrop size to intensity. Trans. Am. Geophys. Union, 24: 452--459. McQueen, I.S., 1963. Development of a hand portable rainfall simulator infiltrometer. U.S. Geol. Surv., Circ. 482. Selby, M.J., 1970. Design of a hand portable rainfall simulating infiltrometer. J. Hydrol. (N.Z.), 9: 117--132. Tricker, A.S., 1977. Infiltration characteristics of a small Pennine catchment in Derbyshire. East Midland G e o g . , 6: 328--336.
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Technical Note [4] THE DESIGN OF A PORTABLE R A I N F A L L SI1VULATOR INFILTROMETER
ALAN S. T R I C K E R
Department of Geography, University of Dundee, Dundee (Great Britain) (Received September 20, 1978; revised and accepted October 11, 1978)
ABSTRACT Tricker, A.S., 1979. The design of a portable rainfall simulator infiltrometer. J. Hydrol., 41 : 143--147. The construction of a hand-portable rainfall simulator infiltrometer in described.
INTRODUCTION
Field measurements of infiltration m a y be attempted using either cylinder or rainfall simulator infiltrometers. The chief advantage of rainfall simulator infiltrometers is that they do give some indication of the rate of infiltration of rainfall as opposed to the ponded-water conditions of cylinders. Disadvantages of rainfall simulator infiltrometers rest with their cost and transportability. Many of these instruments are large, and usually require a vehicle to carry them and their water supply. However, Adams et al. (1957), McQueen (1963) and Selby (1970) have all produced hand-portable rainfall simulating infiltrometers which produce rainfall by the drip-screen method. In the design of Adams et al. drops were produced on the ends of glass capillary tubes. This was considered t o o delicate for field use, and so the drop production m e t h o d of McQueen was tested. A capillary hole (0.8 mm) was drilled through a piece of Perspex ® , 1.0 cm thick. A short length of 24 gauge (0.58 mm) wire was suspended through it b y bending each end of the wire. A small counter-sink drilled at the base of the hole prevented drops flowing across the Perspex surface, and together with the pin, provided an area for each drop to attach and grow. A 2.0 cm diameter disc of Perspex with a single capillary hole drilled through and m o u n t e d in a 5 cm length of polythene tube formed the individual "dropper-unit" from which characteristics of the simulated rainfall were determined. The diameter of raindrops produced was 2.83 ram. However, the diameter was found to be sensitive to any impurities in the water which may reduce surface tension effects, and thus reduce the
144
growth period of drops. Hence the cleanliness of the dropper unit was of paramount importance. Measurements of the fall velocity drops was made using a photographic technique described by Bryan (1967). It was found t h a t the average velocity attained was 5.1 m/s with a fall of 1.5 m. Drops of 2.83 mm diameter, falling at this speed would have the same energy per unit of precipitation as raindrops of 1.4 m m diameter. From the data of Laws and Parsons (1943) this would be the average drop size of rainfall of 2.5 m m / h intensity. INFILTROMETER DESIGN
A compact, portable rainfall simulator infiltrometer was constructed on the basis of a 1.5 m fall height {Fig. 1). The instrument is best described within the framework of its component parts: (a) Drop-producing chamber (Fig. 2). The top and base of this chamber were constructed of 1 cm thick Perspex plate. The Perspex cylinder separating them had an external diameter of 15 cm, and was 12.5 cm deep. The base was cemented into the cylinder, although the top was detachable and had an air hole drilled through the handle. In the base 91 holes were drilled and countersunk in the fasion already described for the "single dropper unit". A tube of 0.8 cm diameter at the side of the cylinder, next to the base allowed water
Fig. 1. The portable rainfall simulator infiltrometer.
145
Fig. 2. The reservoir unit of the rainfall simulator infiltrometer, showing water drops forming at the base of counter-sunk capillary tubes. to e n ter the chamber. A measuring scale was attached to the side of the cylinder to enable changes in the head of water over the capillary tubes to be measured. (b) Float chamber. This was utilised to maintain a constant water head over the capillary tubes. It was constructed o f a 10 cm length of Perspex ® tube, 5 cm in internal diameter, with walls of 1.2 cm thick. The base of the chamber was milled o u t o f Delrin ® (a non-absorbent n y l o n derivative), t hrough which two pipes passed. One pipe was a direct link with the drop producing cylinder, while the o t h e r linked the float chamber and the reservoir. Within the latter tube a stainless steel needle valve was em be dd ed, which was also c o n n e c t e d to a float of ex p an d ed p o l y s t y r e n e (Fig. 3). T he needle valve prevented furt her flow o f water to the d r o p producing chamber once a pre-determined level had been attained. In this m anner a constant head of water could be maintained above the capillary tubes and thus a uni form intensity of rainfall produced. A threaded rod attached to the t op of the float chamber allowed it to be raised or lowered, and in this way, a range o f rainfall intensities could be produced. (c) Reservoir. This consisted of a 5 1 bot t l e m o u n t e d 8 cm above the dropproducing chamber and calibrated in centimetres head of water per unit area o f the plot (15 cm diameter). (d) Wind shield and support. T he Perspex wind shield was 1.35 m long,
146
Fig. 3. The float-chamber control mechanism of the rainfall simulator infiltrometer. and had the same internal diameter as the drop-producing chamber. A PVC ring attached to the base prevented splintering, and a similar ring at the top allowed the drop-producing unit to be slotted into place. The whole instrument was supported by tripod legs braized on to a steelring around the wind shield. A small bubble-level attached to the top of one of the legs allowed the instrument to be erected vertically. (e) Base and runoff collector. The basal cylinder had an internal diameter of 15.0 cm. Angled side-walls prevented any water loss by splash. R u n o f f was removed from the plot to calibrated accumulator tubes through flexible polythene tubes using a small hand-pump. FIELD USE A description of the field use of the infiltrometer follows readily from the description of its manufacture. In the initial tests carried out with the infiltrometer, the plot border was driven 5 cm into the soil in order to allow comparison with infiltration capacities measured by cylinder infiltrometers that were installed in the same manner (Tricker, 1977). At other times, to avoid disturbance of the ground surface, the plot border could be sealed to the ground surface using clay (Selby, 1970). After the plot border and r u n o f f pipe are installed the instrument is erected vertically over the plot, using the bubble-level attached, and the height of the rain producing plate above
147
the ground is adjusted using the tripod legs. A gap of 2.0 cm is left between the top rim of the plot border and the b o t t o m o f the windshield. A galvanished metal tray is inserted in this gap in order to intercept the rainfall until the level in the drop-producing chamber is reached (1--2 min) which will give the required rainfall intensity. At this time the operator slides the galvanised tray clear of the plot, notes the levels in the reservoir and drop-producing chamber, and starts the hand-operated r u n o f f pump. Readings of water levels in the reservoir, chamber and r u n o f f accumulation tubes are taken every five minutes. For each time interval the infiltration rate is calculated from: f=AP-AR-+AC where f = mass infiltration (cm/h); AP = change in reservoir level (cm/h); AR = change in r u n o f f a m o u n t (cm/h); and AC = change in chamber level (cm/h). CONCLUSIONS
The rainfall simulator infiltrometer described can be carried, set up, and operated conveniently by one man and offers a fast, economical means of measuring infiltration capacity. It requires only 1.5--2 man hours per measurement, and thus 3--4 measurements m a y be accomplished in a day. ACKNOWLEDGEMENTS
This work was accomplished while the author held a N.E.R.C. Research Studentship at the University of Sheffield. The instrument was constructed by Mr. C. Fletcher of the Geography Department Workshop.
REFERENCES Adams, J.E., Kirkham, D. and Nielsen, A., 1957. A portable rainfall simulating infiltrometer and physical measurements of soil in place. Soil Sci. Soc. Am., Proc., 21: 473--477. Bryan, R., 1967. The erodibility of some Peak District soils. Ph.D. Thesis, University of Sheffield, Sheffield (unpublished). Laws, J.O. and Parsons, D.A., 1943. The relation of raindrop size to intensity. Trans. Am. Geophys. Union, 24: 452--459. McQueen, I.S., 1963. Development of a hand portable rainfall simulator infiltrometer. U.S. Geol. Surv., Circ. 482. Selby, M.J., 1970. Design of a hand portable rainfall simulating infiltrometer. J. Hydrol. (N.Z.), 9: 117--132. Tricker, A.S., 1977. Infiltration characteristics of a small Pennine catchment in Derbyshire. East Midland G e o g . , 6: 328--336.