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A mechanical balance-type lysimeter for use in remote environments
Agricultural Meteorology, 13(1974) 253--258 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A MECHANICAL BALANCE-TYPE LYSIMETER FOR USE IN REMOTE
ENVIRONMENTS
E. F. LEDREW and J. C. EMERICK
Institute of Arctic and Alpine Research, Department of Geography, University of Colorado, Boulder, Colo. (U.S.A.) Institute of Arctic and Alpine Research, Department of Biology, University of Colorado, Boulder, Colo. (U.S.A.) (Accepted for publication March 12, 1974)
ABSTRACT LeDrew, E. F. and Emerick, J. C., 1974. A mechanical balance-type lysimeter for use in remote environments. Agric. Meteorol., 13: 253--258. A simple weighing lysimeter has been designed and tested in the Alpine tundra environment of the Colorado Front Range. Features of the instrument include lightweight, easy field assembly and maintenance free operation. The lysimeter is unaffected by ambient temperatures and has electrical controls and data output, making remote operation possible. Field tests show that the data obtained are reliable and compare favourably with published results. Comparative tests show that a hydrostatic lysimeter is unacceptable for use in Alpine tundra environments.
INTRODUCTION Moisture a n d e n e r g y b u d g e t studies in r e m o t e l o c a t i o n s are f r e q u e n t l y h a m p e r e d b y difficulties o f assessing e v a p o t r a n s p i r a t i o n rates. This n o t e describes the design o f a simple weighing l y s i m e t e r d e v e l o p e d t o satisfy this r e q u i r e m e n t f o r a p a r t i c u l a r investigation o n the Alpine t u n d r a o f N i w o t Ridge, C o l o r a d o ( 3 , 6 0 0 m). DESIGN AND OPERATION T h e s t u d y site i m p o s e d several c o n s t r a i n t s u p o n the c o n s t r u c t i o n o f a lysimeter. (1) T h e soils are t h i n a n d overlie a r o c k y base; the d e p t h o f t h e soil m o n o lith t o be w e i g h e d m u s t be less t h a n 0.6 m. (2) T h e i n s t r u m e n t m u s t be insensitive to t e m p e r a t u r e e f f e c t s since t h e m e a n diurnal t e m p e r a t u r e v a r i a t i o n is 6°C (Barry, 1973). In t h e g r o w i n g season this range m a y e x c e e d 10°C.
254
(3} The lysimeter must be light, relatively easy to assemble in the field and free f r o m maintenance. (4) The instrument should be designed for rem ot e operation and, since several simultaneous measurements over a large area are required, the data o u t p u t and measurement control should be electrical. The lysimeter shown in Fig.1 meets these constraints. The basic design is that of an arm balance. The soil monolith is supported from a stainless steel
with
chonneJ$
for
\
~
~ootenfiorneter
chain
m~cfolwitch
\
she~fer J
co
,.~,~h,.
I
potentiometer
'
I
p .... io,ioo ,* ..... i, ~
. . . . . . t~on.
~
~ ~
li Ij
J fI
L
_ J
[
Fig.1. A mechanical-balance lysimeter.
beam 1 m long with a thickness of 2.5 cm to ensure rigidity. The beam rests on a fulcrum made from a cold chisel. A lead counterweight is positioned to balance the soil monolith when soil moisture tension is a minimum. Thus, when evapotranspiration is at the potential stage, a m a x i m u m a m o u n t of weight must be added to balance the soil sample. As evapotranspiration proceeds, weight is removed at each observation until equilibrium is reached. The removable weight is a length of chain stored on a drum. A precision D.C. m o t o r with a reduction gear is activated to rotate the drum and remove weight from, or add it to, the balance arm. The drum is channeled to ensure that the length of chain added per revolution is constant. The n u m b e r of revolutions of the drum is measured as an electrical resistance by a ten-turn, 5 K ~2 p o t e n t i o m e t e r . Weight added is determined from calibration curves derived for each instrument. Before each observation the drum is r et urned to a reference position, as determined from the p o t e n t i o m e t r i c reading, by reversing the polarity of the motor. The m o t o r is then activated with normal polarity and weight is added. A microswitch is rigidly positioned under the balance arm near the chain
255 attachment; the switch closes when the balance reaches equilibrium. The D.C. motor is then shut off automatically and the drum position can be determined when convenient by reading the potentiometer with an Ohm-meter. In this manner the weight change of the soil monolith between observations, which corresponds to the difference in weight added, can be measured remotely through electrical observation and control. CALIBRATION The lysimeter was calibrated in the field by adding laboratory weights to the soil monolith and recording the change in electrical resistance when the system is returned to equilibrium. By adjusting the position of the counterweight the initial equilibrium position of the system could be set at different rotational positions of the drum and the linearity of the weighing mechanism could be evaluated. For the first instrument 1 g of weight change on a surface area of 400 cm 2 equaled 70 ~2. For t w e n t y calibration tests the reproducibility of this factor was + 5% for the entire range of initial drum positions. The performance is almost linear and we expect insignificant mechanical errors. The smallest weight change measured during the field season was 5 g over a 2-h period. This is equivalent to a vapor flux of 1.6-10 .6 g cm -2 sec -1 . FIELD INSTALLATION The soil monolith was contained in a 14-kg pail and the ground cavity was lined with a 19-1itre drum. This design follows that of Courtin and Bliss (1971) for a hydrostatic lysimeter, Webber et al. (1974) have shown that 80--98% of the below-ground biomass is contained within the upper 25 cm of soil. Therefore there is minimal disturbance of root structure with a container of these dimensions. Using this arrangement Courtin and Bliss estimated the gap between the sample and the surrounding turf to be 30% of the lysimeter area. The percolation reservoir is based on their specifications also. The instrument was partially housed in a sheet metal case to reduce wind effects on the weighing mechanism and counterweight. A sheet metal shield was placed immediately upwind of the monolith during the observation period to minimize wind vibration. RELIABILITY OF MEASUREMENT The obvious objection to a field lysimeter of this nature is the small size of the monolith. A larger surface area would include more vegetation units and probably represent the spatial distribution of vegetation types more realistically. Small-scale turbulence created at the edge will affect atmospheric transport over a large fraction of a surface if the diameter is small. Similarly, heat conduction through the metal walls could affect soil moisture transport
256 through a large proportion of the soil sample. A deeper unit would have a greater capacity for ground water storage, possibly a significant source for recharging of the surface layers as they dry out. Konstantinov (1963) cites several Russian studies of the effect of lysimeter size on evapotranspiration. Both for regions of moisture excess and deficit the minimum recommended surface area is 500 cm 2 and the minimum depth is 0.5 m. The moisture and thermal regimes of the soil may be altered in samples smaller than these critical dimensions. A comparison of a lysimeter with these minimum dimensions and one with a depth of 2 m and a surface area of 5 m 2 shows very small differences up to an accumulated water loss after saturation of 15 cm. At this point the difference is approximately 15%. The dimensions of the 14-kg pail used in this study are close to these critical limits and the maximum water loss after saturation was 6 cm at the Niwot Ridge site. We expect that the LeDrew--Emerick lysimeter should yield reasonably representative measures of evapotranspiration. As an indirect test of this statement for actual field conditions, leaf water potentials of representative vegetation types within and outside of the lysimeter were measured on August 25, 1973, following a period of eleven days with no measurable precipitation. If recharging of the surface layers from deep storage of meltwater significantly alters the evapotranspiration, this effect would n o t be masked by temporary surface storage of precipitation. The basic assumption is that a large difference in leaf water potential between the two samples would reflect a difference in the rates of evapotranspiration for August 25. Since the stage of leaf development is dictated by the seasonal history of moisture relations, this would probably indicate a difference in the rates of evapotranspiration throughout the growing season also. The mean leaf water potential for the lysimeter sample was - 1 8 bars and for the natural environment it was - 1 7 bars. The difference is insignificant, thus giving confidence in the reliability of our measurements. The seasonal trend of evapotranspiration based on lysimeter measurements seems comparable with published data. Potential evapotranspiration was computed for the 4-h period centered on solar noon using the m e t h o d of Priestley and Taylor (1972). Gradients of temperature and specific humidity were measured between 20 and 40 cm at 15-min intervals and averaged for the period. Net radiation at 1 m and soil heat flux at -2.5 cm were measured at hourly intervals and averaged for the period. Lysimeter data for this 4-h period were used to compute accumulated evapotranspiration and the ratio of actual to potential evapotranspiration for August 1973. The previous week had 1.9 cm of precipitation which is close to the criterion for saturation adopted by Priestley and Taylor (1972). The data are presented in Fig.2, with a best-fit line drawn by eye. The relationship resembles that given by Priestley and Taylor {1972) for a similar range of data from Aspendale and Katherine, Australia and O'Neill, Nebraska. The initial ratio is less than unity which probably reflects the increased leaf resistance at solar noon, evident on a diurnal graph of lysimeter observations. The leaf water potential of - 1 8
257
bars at a ratio of 0.15 compares favourably with similar data from Phoenix ( - 1 8 bars, Priestley and Taylor, 1972) and with values given by Monteith (1965) for a plot of pepper, sunflower and cotton ( - 1 6 bars). COMPARISON OF THE BALANCE-TYPE AND HYDROSTATIC LYSIMETER
A hydrostatic lysimeter has been designed for tundra environments by Courtin and Bliss (1971). Three lysimeters were built from their plans and installed during the 1972 growing season in the study plot on Niwot Ridge. Z
I-¢1.
.9
U
Z n~ I0 a_
.6 LU .3 Z LU I-0 (3_
,3
.J
ACCUMULATED EVAPOTRANSPIRATION (CM}
Fig.2. The evapotranspiration regime of an Alpine tundra from August 2 to August 25, 1973.
As suggested by the authors, one was sealed and used as a d u m m y to correct for environmental changes. With the highly variable winds experienced in the Alpine tundra satisfactory readings of the monometer tubes were impossible as a result of a pronounced pitot effect. In 1973 Tygon tubing was attached to the open end of the monometer tube. The free end of the tubing was buried in rock at ground level to minimize the wind effect. Readings from all three lysimeters were taken at 08h00 and 14h00 L.S.T. to correspond with readings taken from the LeDrew--Emerick balance-type lysimeter. After adjusting the two hydrostatic lysimeters with the dummy, a linear regression of the two samples accounted for only 16% of the variance. The sign of the weight change was often opposite for the two lysimeters with no apparent dependence on any environmental factors. A similar comparison of one hydrostatic lysimeter with the balance-type lysimeter accounted for 36% of the variance but the F statistic was not significant. The slope of the relationship could be zero. The poor comparison of two identical hydrostatic lysimeters suggests that this design gives unreliable results in the Alpine tundra environment. Apparently
258 t h e y c a n n o t be successfully calibrated t o c o m p e n s a t e for e n v i r o n m e n t a l effects on the mechanism. Since the h y d r o s t a t i c fluid is c o n t a i n e d in a small inner tube, it is possible t h a t air could be t r a p p e d and the unit w o u l d act as an a n e r o i d b a r o m e t e r . With d i f f e r e n t volumes of air enclosed, the e f f e c t w o u l d be d i f f e r e n t for each lysimeter. With d i f f e r e n t volumes o f fluid, the t e m p e r a t u r e effects w o u l d also be d i f f e r e n t for each lysimeter. The d u m m y w o u l d be ineffective as a c o r r e c t i o n , and m a y increase the error. CONCLUSIONS A weighing l y s i m e t e r has been designed f o r o p e r a t i o n at r e m o t e locations. A survey o f the literature suggests t h a t the size (surface area o f 4 0 0 cm 2, d e p t h o f 35 cm) should be sufficient to p r o d u c e reliable m e a s u r e m e n t s o f the m o i s t u r e fluxes. Field testing and c o m p a r i s o n with published d a t a lends confidence to the m e a s u r e m e n t s . A c o m p a r i s o n o f t w o Courtin--Bliss h y d r o static lysimeters shows t h a t t h e y are unreliable in Alpine t u n d r a e n v i r o n m e n t s and t h a t t h e y c a n n o t be calibrated to c o r r e c t for e n v i r o n m e n t a l influences. ACKNOWLEDGEMENTS Financial s u p p o r t for this p r o j e c t was provided b y the Council on Research and Creative Work, University o f Colorado, and the I n t e r n a t i o n a l Biological Program, T u n d r a Biome (NSF G r a n t No. G V - 2 9 3 5 0 to Dr. J. D. Ives). Dr. P. J. Webber assisted in the d e v e l o p m e n t o f the design. Dr. R. G. Barry and Dr. P. J. Webber read the preliminary d r a f t and their c o n s t r u c t i v e criticism is appreciated. The leaf w a t e r p o t e n t i a l m e a s u r e m e n t s were m a d e b y D. J o h n s o n of Utah State University. REFERENCES Barry, R. G., 1973. A climatological transect along the east slope of the Front Range, Colorado. Arct. Alp. Res., 5: 89--110. Courtin, G. M. and Bliss, L. C., 1971. A hydrostatic lysimeter to measure evapotranspiration under remote field conditions. Arct. Alp. Res., 3: 81--89. Konstantinov, A. R., 1963. Evaporation in Nature. Translated from Russian by Israel Program for Scientific Translations, Jerusalem, 1966, 523 pp. Monteith, J. L., 1965. Evaporation and environment. Syrup. Soc. Exptl. Biol., 19: 205--234. Priestley, C. H. B. and Taylor, R. J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev., 100: 81--92. Webber, P. J., Ebert May, D. and Wielgolaski, F. E., 1974. The Distribution and Adaptive Strategy of Belowground Material in the Alpine Vegetation of Niwot Ridge, Colorado. In press.
A MECHANICAL BALANCE-TYPE LYSIMETER FOR USE IN REMOTE
ENVIRONMENTS
E. F. LEDREW and J. C. EMERICK
Institute of Arctic and Alpine Research, Department of Geography, University of Colorado, Boulder, Colo. (U.S.A.) Institute of Arctic and Alpine Research, Department of Biology, University of Colorado, Boulder, Colo. (U.S.A.) (Accepted for publication March 12, 1974)
ABSTRACT LeDrew, E. F. and Emerick, J. C., 1974. A mechanical balance-type lysimeter for use in remote environments. Agric. Meteorol., 13: 253--258. A simple weighing lysimeter has been designed and tested in the Alpine tundra environment of the Colorado Front Range. Features of the instrument include lightweight, easy field assembly and maintenance free operation. The lysimeter is unaffected by ambient temperatures and has electrical controls and data output, making remote operation possible. Field tests show that the data obtained are reliable and compare favourably with published results. Comparative tests show that a hydrostatic lysimeter is unacceptable for use in Alpine tundra environments.
INTRODUCTION Moisture a n d e n e r g y b u d g e t studies in r e m o t e l o c a t i o n s are f r e q u e n t l y h a m p e r e d b y difficulties o f assessing e v a p o t r a n s p i r a t i o n rates. This n o t e describes the design o f a simple weighing l y s i m e t e r d e v e l o p e d t o satisfy this r e q u i r e m e n t f o r a p a r t i c u l a r investigation o n the Alpine t u n d r a o f N i w o t Ridge, C o l o r a d o ( 3 , 6 0 0 m). DESIGN AND OPERATION T h e s t u d y site i m p o s e d several c o n s t r a i n t s u p o n the c o n s t r u c t i o n o f a lysimeter. (1) T h e soils are t h i n a n d overlie a r o c k y base; the d e p t h o f t h e soil m o n o lith t o be w e i g h e d m u s t be less t h a n 0.6 m. (2) T h e i n s t r u m e n t m u s t be insensitive to t e m p e r a t u r e e f f e c t s since t h e m e a n diurnal t e m p e r a t u r e v a r i a t i o n is 6°C (Barry, 1973). In t h e g r o w i n g season this range m a y e x c e e d 10°C.
254
(3} The lysimeter must be light, relatively easy to assemble in the field and free f r o m maintenance. (4) The instrument should be designed for rem ot e operation and, since several simultaneous measurements over a large area are required, the data o u t p u t and measurement control should be electrical. The lysimeter shown in Fig.1 meets these constraints. The basic design is that of an arm balance. The soil monolith is supported from a stainless steel
with
chonneJ$
for
\
~
~ootenfiorneter
chain
m~cfolwitch
\
she~fer J
co
,.~,~h,.
I
potentiometer
'
I
p .... io,ioo ,* ..... i, ~
. . . . . . t~on.
~
~ ~
li Ij
J fI
L
_ J
[
Fig.1. A mechanical-balance lysimeter.
beam 1 m long with a thickness of 2.5 cm to ensure rigidity. The beam rests on a fulcrum made from a cold chisel. A lead counterweight is positioned to balance the soil monolith when soil moisture tension is a minimum. Thus, when evapotranspiration is at the potential stage, a m a x i m u m a m o u n t of weight must be added to balance the soil sample. As evapotranspiration proceeds, weight is removed at each observation until equilibrium is reached. The removable weight is a length of chain stored on a drum. A precision D.C. m o t o r with a reduction gear is activated to rotate the drum and remove weight from, or add it to, the balance arm. The drum is channeled to ensure that the length of chain added per revolution is constant. The n u m b e r of revolutions of the drum is measured as an electrical resistance by a ten-turn, 5 K ~2 p o t e n t i o m e t e r . Weight added is determined from calibration curves derived for each instrument. Before each observation the drum is r et urned to a reference position, as determined from the p o t e n t i o m e t r i c reading, by reversing the polarity of the motor. The m o t o r is then activated with normal polarity and weight is added. A microswitch is rigidly positioned under the balance arm near the chain
255 attachment; the switch closes when the balance reaches equilibrium. The D.C. motor is then shut off automatically and the drum position can be determined when convenient by reading the potentiometer with an Ohm-meter. In this manner the weight change of the soil monolith between observations, which corresponds to the difference in weight added, can be measured remotely through electrical observation and control. CALIBRATION The lysimeter was calibrated in the field by adding laboratory weights to the soil monolith and recording the change in electrical resistance when the system is returned to equilibrium. By adjusting the position of the counterweight the initial equilibrium position of the system could be set at different rotational positions of the drum and the linearity of the weighing mechanism could be evaluated. For the first instrument 1 g of weight change on a surface area of 400 cm 2 equaled 70 ~2. For t w e n t y calibration tests the reproducibility of this factor was + 5% for the entire range of initial drum positions. The performance is almost linear and we expect insignificant mechanical errors. The smallest weight change measured during the field season was 5 g over a 2-h period. This is equivalent to a vapor flux of 1.6-10 .6 g cm -2 sec -1 . FIELD INSTALLATION The soil monolith was contained in a 14-kg pail and the ground cavity was lined with a 19-1itre drum. This design follows that of Courtin and Bliss (1971) for a hydrostatic lysimeter, Webber et al. (1974) have shown that 80--98% of the below-ground biomass is contained within the upper 25 cm of soil. Therefore there is minimal disturbance of root structure with a container of these dimensions. Using this arrangement Courtin and Bliss estimated the gap between the sample and the surrounding turf to be 30% of the lysimeter area. The percolation reservoir is based on their specifications also. The instrument was partially housed in a sheet metal case to reduce wind effects on the weighing mechanism and counterweight. A sheet metal shield was placed immediately upwind of the monolith during the observation period to minimize wind vibration. RELIABILITY OF MEASUREMENT The obvious objection to a field lysimeter of this nature is the small size of the monolith. A larger surface area would include more vegetation units and probably represent the spatial distribution of vegetation types more realistically. Small-scale turbulence created at the edge will affect atmospheric transport over a large fraction of a surface if the diameter is small. Similarly, heat conduction through the metal walls could affect soil moisture transport
256 through a large proportion of the soil sample. A deeper unit would have a greater capacity for ground water storage, possibly a significant source for recharging of the surface layers as they dry out. Konstantinov (1963) cites several Russian studies of the effect of lysimeter size on evapotranspiration. Both for regions of moisture excess and deficit the minimum recommended surface area is 500 cm 2 and the minimum depth is 0.5 m. The moisture and thermal regimes of the soil may be altered in samples smaller than these critical dimensions. A comparison of a lysimeter with these minimum dimensions and one with a depth of 2 m and a surface area of 5 m 2 shows very small differences up to an accumulated water loss after saturation of 15 cm. At this point the difference is approximately 15%. The dimensions of the 14-kg pail used in this study are close to these critical limits and the maximum water loss after saturation was 6 cm at the Niwot Ridge site. We expect that the LeDrew--Emerick lysimeter should yield reasonably representative measures of evapotranspiration. As an indirect test of this statement for actual field conditions, leaf water potentials of representative vegetation types within and outside of the lysimeter were measured on August 25, 1973, following a period of eleven days with no measurable precipitation. If recharging of the surface layers from deep storage of meltwater significantly alters the evapotranspiration, this effect would n o t be masked by temporary surface storage of precipitation. The basic assumption is that a large difference in leaf water potential between the two samples would reflect a difference in the rates of evapotranspiration for August 25. Since the stage of leaf development is dictated by the seasonal history of moisture relations, this would probably indicate a difference in the rates of evapotranspiration throughout the growing season also. The mean leaf water potential for the lysimeter sample was - 1 8 bars and for the natural environment it was - 1 7 bars. The difference is insignificant, thus giving confidence in the reliability of our measurements. The seasonal trend of evapotranspiration based on lysimeter measurements seems comparable with published data. Potential evapotranspiration was computed for the 4-h period centered on solar noon using the m e t h o d of Priestley and Taylor (1972). Gradients of temperature and specific humidity were measured between 20 and 40 cm at 15-min intervals and averaged for the period. Net radiation at 1 m and soil heat flux at -2.5 cm were measured at hourly intervals and averaged for the period. Lysimeter data for this 4-h period were used to compute accumulated evapotranspiration and the ratio of actual to potential evapotranspiration for August 1973. The previous week had 1.9 cm of precipitation which is close to the criterion for saturation adopted by Priestley and Taylor (1972). The data are presented in Fig.2, with a best-fit line drawn by eye. The relationship resembles that given by Priestley and Taylor {1972) for a similar range of data from Aspendale and Katherine, Australia and O'Neill, Nebraska. The initial ratio is less than unity which probably reflects the increased leaf resistance at solar noon, evident on a diurnal graph of lysimeter observations. The leaf water potential of - 1 8
257
bars at a ratio of 0.15 compares favourably with similar data from Phoenix ( - 1 8 bars, Priestley and Taylor, 1972) and with values given by Monteith (1965) for a plot of pepper, sunflower and cotton ( - 1 6 bars). COMPARISON OF THE BALANCE-TYPE AND HYDROSTATIC LYSIMETER
A hydrostatic lysimeter has been designed for tundra environments by Courtin and Bliss (1971). Three lysimeters were built from their plans and installed during the 1972 growing season in the study plot on Niwot Ridge. Z
I-¢1.
.9
U
Z n~ I0 a_
.6 LU .3 Z LU I-0 (3_
,3
.J
ACCUMULATED EVAPOTRANSPIRATION (CM}
Fig.2. The evapotranspiration regime of an Alpine tundra from August 2 to August 25, 1973.
As suggested by the authors, one was sealed and used as a d u m m y to correct for environmental changes. With the highly variable winds experienced in the Alpine tundra satisfactory readings of the monometer tubes were impossible as a result of a pronounced pitot effect. In 1973 Tygon tubing was attached to the open end of the monometer tube. The free end of the tubing was buried in rock at ground level to minimize the wind effect. Readings from all three lysimeters were taken at 08h00 and 14h00 L.S.T. to correspond with readings taken from the LeDrew--Emerick balance-type lysimeter. After adjusting the two hydrostatic lysimeters with the dummy, a linear regression of the two samples accounted for only 16% of the variance. The sign of the weight change was often opposite for the two lysimeters with no apparent dependence on any environmental factors. A similar comparison of one hydrostatic lysimeter with the balance-type lysimeter accounted for 36% of the variance but the F statistic was not significant. The slope of the relationship could be zero. The poor comparison of two identical hydrostatic lysimeters suggests that this design gives unreliable results in the Alpine tundra environment. Apparently
258 t h e y c a n n o t be successfully calibrated t o c o m p e n s a t e for e n v i r o n m e n t a l effects on the mechanism. Since the h y d r o s t a t i c fluid is c o n t a i n e d in a small inner tube, it is possible t h a t air could be t r a p p e d and the unit w o u l d act as an a n e r o i d b a r o m e t e r . With d i f f e r e n t volumes of air enclosed, the e f f e c t w o u l d be d i f f e r e n t for each lysimeter. With d i f f e r e n t volumes o f fluid, the t e m p e r a t u r e effects w o u l d also be d i f f e r e n t for each lysimeter. The d u m m y w o u l d be ineffective as a c o r r e c t i o n , and m a y increase the error. CONCLUSIONS A weighing l y s i m e t e r has been designed f o r o p e r a t i o n at r e m o t e locations. A survey o f the literature suggests t h a t the size (surface area o f 4 0 0 cm 2, d e p t h o f 35 cm) should be sufficient to p r o d u c e reliable m e a s u r e m e n t s o f the m o i s t u r e fluxes. Field testing and c o m p a r i s o n with published d a t a lends confidence to the m e a s u r e m e n t s . A c o m p a r i s o n o f t w o Courtin--Bliss h y d r o static lysimeters shows t h a t t h e y are unreliable in Alpine t u n d r a e n v i r o n m e n t s and t h a t t h e y c a n n o t be calibrated to c o r r e c t for e n v i r o n m e n t a l influences. ACKNOWLEDGEMENTS Financial s u p p o r t for this p r o j e c t was provided b y the Council on Research and Creative Work, University o f Colorado, and the I n t e r n a t i o n a l Biological Program, T u n d r a Biome (NSF G r a n t No. G V - 2 9 3 5 0 to Dr. J. D. Ives). Dr. P. J. Webber assisted in the d e v e l o p m e n t o f the design. Dr. R. G. Barry and Dr. P. J. Webber read the preliminary d r a f t and their c o n s t r u c t i v e criticism is appreciated. The leaf w a t e r p o t e n t i a l m e a s u r e m e n t s were m a d e b y D. J o h n s o n of Utah State University. REFERENCES Barry, R. G., 1973. A climatological transect along the east slope of the Front Range, Colorado. Arct. Alp. Res., 5: 89--110. Courtin, G. M. and Bliss, L. C., 1971. A hydrostatic lysimeter to measure evapotranspiration under remote field conditions. Arct. Alp. Res., 3: 81--89. Konstantinov, A. R., 1963. Evaporation in Nature. Translated from Russian by Israel Program for Scientific Translations, Jerusalem, 1966, 523 pp. Monteith, J. L., 1965. Evaporation and environment. Syrup. Soc. Exptl. Biol., 19: 205--234. Priestley, C. H. B. and Taylor, R. J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev., 100: 81--92. Webber, P. J., Ebert May, D. and Wielgolaski, F. E., 1974. The Distribution and Adaptive Strategy of Belowground Material in the Alpine Vegetation of Niwot Ridge, Colorado. In press.