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Soil environment and the hydrometeorological mosaic
Agricultural Meteorology, 18(1977) 425--433 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
SOIL ENVIRONMENT AND THE HYDROMETEOROLOGICAL MOSAIC*
IAN REID
Birkbeck College, University of London, London (Great Britain) (Received March 9, 1977; accepted May 10, 1977)
ABSTRACT Reid, I., 1977. Soil environment and the hydrometeorological mosaic. Agric. Meteorol., 18: 425--433. Seasonal changes in the physical status of the soil are largely a response to progressive changes in magnitude and balance of hydrometeorological variables. Soil temperature may, other things being equal, show a comparatively simple response, the spatial inhomogeneity resulting from differences in ground slope and aspect. In the case of resident water, the soil exerts its own set of limiting conditions. As a result, the soil moisture pattern need not reflect the hydrometeorological mosaic. The results of a year's experiments at Caydell, North York Moors, England, are presented. INTRODUCTION
Seasonal adjustment in the magnitude and mutual balance of the hydrometeorological variables should be reflected in a changing physical status of the soil. Of the control variables, solar radiation plays a dominant role (Davenport, 1967), but energy exchanges at the ground surface are determined in part by local slope. Since the form of the land is complex it follows that there will exist an hydrometeorological mosaic at least as complicated. This in itself merely defines a pattern of potential process-response and is rendered simple by ignoring the additional complications that attend vegetational diversity. Such a situation is the setting for the hydrometeorological experiments at Caydell, North York Moors. Within this small valley, observations show that response variables such as soil temperature demonstrate spatial inhomogeneities which approach those predicted by the potential model. Other response variables, principally soil water content and actual evapotranspiration, diverge significantly. The soil exerts its own set of limiting conditions. Because of this the resulting spatial pattern of soil moisture content need not conform to the hydrometeorological mosaic. * A pattern of contiguous land units, differentiated hydrometeorologically by ground slope and aspect.
426
This is particularly true at a time of the year when crop growth rates are critical. Prediction of subtle aspect-controlled differences in spring soil temperatures and the adjustment of field management will benefit germination and protect seedlings, especially where crops are sensitive or where species have been newly introduced to British farming. At a later stage, an ability to make short-term forecasts of changing summer soil conditions becomes vital to the rational deployment of artificial soil water recharge. Given increasing use of sprinkler irrigation in low rainfall areas and a growing awareness of limited water resource, greater precision of estimated water-needs is desirable on farms that cover a variety of slope aspects. The influence of ground slope and azimuth upon soil environmental response to hydrometeorological variables was first studied by Wollny (1878). Since then the influence of slope aspect has been developed by two main schools: the German micro meteorologists, which have included Kerner (1891), Held (1941), Grunow (1953), Heigel (1958), Hartmann et al. (1959), and, of course, Geiger (1966); and the American ecologists led by Shreve (1924), but including Gail (1921), Bates (1923}, Cottle (1932), Potzger (1939), Wolfe et al. (1943), Shanks and Norris (1950), Parker (1952), and Cantlon (1953). More recently, the Japanese have been represented by Sakanoue (1959), and Watanabe (1959), while a new wave of interest has been shown by North American hydrologists and micrometeorologists, (Swift and Van Bavel, 1961; Lee, 1964; Ferguson et al., 1971; Swift, 1976). The British literature has been less prolific, which may reflect a subconscious feeling that the equable airmass dominated weather of the British Isles dampens down any large aspectinduced inhomogeneities. Nevertheless, it includes contributions from Taylor (1958), Morris and Barry (1963), Curtis (1971}, and Reid (1973a). EXPERIMENTAL GROUND
Caydell is a west--east incision of the Hambleton Hills (Fig.l). Its southand north-facing sideslopes show considerable geomorphological symmetry. The valley drops 100 m from a surrounding plateau which provides an uncomplicated skyline. This simplifies the pattern of solar radiation receipt, and the influence of topography upon airflow. In short, there is a distinct break between plateau and valley environments. Sites were selected on opposing valley sideslopes undifferentiated by geological factors (the local dip is insignificant}, or by vegetation (both are grazed permanent pasture}. Rainfall, soil moisture, soil drainage, and soil temperature were measured at regular weekly intervals from October, 1969 to October, 1970. Rainfall was determined by a network of 1 autographic and 19 nonrecording gauges, soil moisture was recorded for 5 soil depths at 36 sites, soil temperature at 10, 20 and 30 cm at 4 sites and soil drainage in 3 banks of monolith lysimeters. Details of the sampling variance of soil water content, and of the spatial patterns of rainfall induced by slope aspect are given elsewhere (Reid 1973a, b). The thermometers were Meteorological Office pattern
427
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0+54°
,:-~
Caydell
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~
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.
. . . . . .
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Z --- . . . . . . .
°
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0
500
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Fig.1. Index maps of Caydell. Contours extracted from O.S. Sheet SE 58 NW.
bent-stem mercury-in-glass, each calibrated against a National Physics Laboratory certified thermometer. Soil temperatures are 09h00 GMT readings. The site readings axe averaged to give a value for a soil block between 10 cm and 30 cm depth. No record of the diurnal penetration of the heat-wave into the soil was attempted at Caydell, but other results (Keen and Russell, 1921; Smith, 1929; Blanc, 1958) suggest that the 09h00 reading corresponds to a daily minimum at these soil depths. SOIL TEMPERATURE
The thermal character of advected air-masses affects all slopes in a similar manner. Any differentials in soil temperature at sites of different aspect or ground slope are, therefore, largely a reflection of differences in receipt of short-wave radiation. This assumes that soil response (thermal conductivity, specific heat, and so on) at each site is similar. For Caydell, the important soil variables of porosity and organic matter are not significantly different between sites (Reid, 1972). A major factor in the soil heat budget, however, is the accumulated difference in soil water content that develops in summer between south- and north-facing slopes. Its effect upon soil temperature response will be referred to later. The horizon-shadow diagram (Fig.2) illustrates for selected dates the approximate position of maximal sunlight penetration into Caydell relative to the thermometer stations. The south-facing site is always clear of the
428
NST4
...7 .5
NST 2 NST4
f J
s--'-'-
J
1. 2. 3. 4. 5. 6. 7.
October October November December February February March
N
f
13 27 10 22 2 16 2
Fig.2. Mid-day horizon-shadow, valley slope profiles, and soil thermometer sites.
horizon-shadow. NST3 and NST2 receive no direct sunlight for 18 weeks (midOctober to early March) while NST4, further downslope on the north-facing side of the valley, is hidden for approximately 10 weeks. The influence upon soil temperature of increasing time spent in shadow is reflected in a ranking of the intersite correlation coefficients. The south-facing site (SST1) correlates best with NST4, followed by NST3 and NST2. Ranking of coefficients involving NST2 produces an order exactly opposite. At this gross level of analysis, soil temperature follows an expected spatial pattern and is a reasonably predictable response variable of the hydrometeorological mosaic. This is further attested by plotting differences in temperature between aspect as a function of time (Fig.3). There is a progressive increase in differential warming of Caydell slopes in late winter and spring. During the sampling period, this is interrupted in January by a reduction in sunshine associated with an influx of comparatively
429 A. Sunshine and Air Temperature
4°I IINHIIIIHHIIWIm ~o I
~8~
~ 1 ";2o
-5.1
B.
Soil
Temperature
2'0 y = 1.9437,0.3267× 1"5
• 0-0115×2 • 0.0001× :~
r = 0'596
U ,L_.
•
•
p~
05 0"0
,
,
Nov.
D e c . Jan
,
•
Feb
M a r , Apr
May
Jun.
,
dul.
Aug.
•
,
,
Sept. Oct.
Fig.3. D i f f e r e n c e s in soil t e m p e r a t u r e a t SST1 (Ts) a n d N S T 3 (Tn), a n d s u n s h i n e h o u r s (n) a n d air t e m p e r a t u r e (Ta).
warm air and attendant cloud cover. Yet the rise in differences towards a maximum at the spring equinox is not halted. By the beginning of March the north-facing slope has emerged from its winter shade and from then until the solstice comes increasingly under the influence of direct sunlight. Soil temperature differences fall correspondingly. Following the solstice, the slightly moister soil of the north-facing slope, giving greater thermal capacity and perhaps lower thermal diffusivity, ensures that differences in soil temperature remain low even though the sun is now sinking. But differences inevitably increase as the north-facing slope moves towards its winter shade once more. Residual warmth is responsible for the lag in the development of maximum differences between slope in autumn. The north-facing slope soil cools as radiation inputs get smaller, while the south-facing slope soil is merely allowed to cool less rapidly. This difference in behaviour between a u t u m n and spring is emphasized by comparing the out-of-phase nature of equinox and m a x i m u m soil temperature differences in autumn with their coincidence on the warming limb of the annual cycle. The result is an a s y m m e t r y in the general biannual pattern of rise and fall (Fig.3). The limbs of the spring fluctuation are longer than are those of the autumn rise and fall, yet the maximum differences achieved are similar. SOIL WATER
Inequalities in soil water content do not reflect so simply the potential
430
spatial pattern which might derive from differences in solar radiation receipt alone. This is partly a function of the delicate and changing balance of the water-holding and -handling capacities of the soil, and partly due to the differential regulation of actual evapotranspiration under conditions of increasing soil water deficit. Broadly, the pattern is as expected (Fig.4) with greatest differences between aspect occurring in summer. To facilitate direct comparison, current water contents are expressed in ratio with the mean winter soil moisture content since it can be demonstrated (Reid, 1975) that this value is a sensitive indicator of the water-holding and -handling capacities of the soil. Having derived these values for all soil depths and all sampling weeks, the south-facing is subtracted from the north-facing to construct Fig.4. From mid-November until the end of April, the soil water content moves around the winter mean values of both north- and south-facing slopes. There is little hydrometeorologically induced spatial inequality, though if precipitation inputs decrease, as in March, the south-facing slope tends to slight relative dryness. The onset of drought in May terminates significant replenishment of the soil water reservoir which until now has been feeding the evapotranspiration process at rates unhindered by soil moisture deficit. At the same time, potential evaporative demand continues to increase as the sun moves towards the summer solstice. This demand is at all times significantly higher for the southfacing slope. Progression of a drying front into the soil of the south-facing slope initially promotes large differences in soil water content between aspect. However, from the beginning of June, two factors combine to reduce the inequality that has developed. The exaggerated soil moisture deficit of the south-facing slope maintains actual evapotranspiration considerably below the local potential demand. The south-facing slope soil remains, therefore, moister than would be expected. The second factor is that the north-facing slope, having initially suffered smaller water losses at the onset of drought, continues to lose water by evapotranspiration at rates which eventually exceed those of the south-facing slope (Reid, 1973a). Summer spatial inequalities in soil moisture are not of a magnitude expected from the mosaic of hydrometeorological demands. In general, soil--plant systems on south-facing slopes tend to conserve moisture. CONCLUSIONS
Theoretical modelling of control variables in the hydrometeorological mosaic is feasible (Kimball, 1919). Computation of the changing magnitude of actual solar radiation dose for any given slope has been facilitated by the development of suitable algorithms (Swift, 1976), and relies upon the adequacy of available data. More problematical is the modelling of the mosaic response variables. Caydell demonstrates varying degrees of predictability. Soil temperature response is comparatively simple. On the other hand, changing
431 "0
m m
9 0 u
-~
m
q
I .,..=. i
z m u r
n
o
m
m u
m ~ m
m- Z
lww) Fte~u!eu
{ ~ 3 ) qldeO I~0S
0
432
soil water content relies upon the variability in the rates of such processes as actual evapotranspiration which are in part regulated by soil moisture deficit, a negative feedback. To the British farmer, the small aspect controlled differences in minimum soil temperature during spring and autumn determine frost susceptibility. All except two observations of frozen-ground at Caydell indicate comparatively favourable conditions on the south-facing slope. But the phenological advantage gained in spring through higher soil temperatures on south-facing slopes may be more than compensated by the earlier onset of soil drought in early summer.
ACKNOWLEDGEMENTS
The programme of field experiments was carried out while sponsored by the Natural Environment Research Council. Thanks are due to Mr. P. Guthrie who owns a substantial portion of Caydell, and to the Headmaster, Ampleforth School for allowing access to the daily records of their meteorological station.
REFERENCES Bates, C. G., 1923. The transect of a mountain valley. Ecology, 4: 54--62. Blanc, M. L., 1958. The climatological investigation of soil temperature. World Meteorol. Organ. Tech. Note, 20, 30 pp. Cantlon, J. E., 1953. Vegetation and microclimates on north and south slopes of Cushetunk Mountain, New Jersey. Ecol. Monogr., 23: 241--270. Cottle, H. J., 1932. Vegetation on north and south slopes of mountains in south western Texas. Ecology, 13: 121--134. Curtis, L. F., 1971. Soils of Exmoor Forest. Soil Survey of England and Wales, Special Survey, 5, 77 pp. Davenport, D. C., 1967. Variations of evaporation in time and space: 1, study of diurnal changes using evaporimeters and grass lysimeters. J. Hydrol., 5: 312--328. Ferguson, H. L., Cork, H. F., Anderson, R. L., Mastoni, S. and Weisman, B., 1971. Theoretical clear sky effective insolation over a small mountain basin. Clim. Stud., 21, 45 pp. Gall, F. W., 1921. Factors controlling the distribution of Douglas Fir in semi-arid regions of the northwest. Ecology, 2: 281--291. Geiger, R., 1966. The Climate near the Ground. Harvard Univ. Press, Cambridge, (Mass.), (Revised edition), 611 pp. Grunow, J., 1953. Niederschlagsmessungen am Hang. Meteorologische Rundsch. 6: 85--91. Hartmann, F. K., Van Eimern, J. and Jahn, G., 1959. Untersuchungen reliefbedingter kleinklimatischer Fragen in Gel~/ndequerschnitten der hochmontanen und montanen Stufe der Mittel- und S~dwestharzes. Ber. Dtsch. Wetterdienster, 50, 39 pp. Heigel, K., 1958. Ergebnisse von Bodenfeuchtemessungen mit Gipsscheibenelektroden. Meteorol. Rundsch., 11: 92--96. Held, J. R., 1941. Temperatur und Relative Feuchtigkeit auf Sonn und Schattenseite in einem Alpenl~ngstal. Meteorol. Z.,58: 398--404. Keen, B. A. and Russell, E. J., 1921. The factors determining soil temperature. J. Agric. Sci., 11: 211--239.
433 Kerner, A., 1891. Die/~nderung der Bodentemperatur mit der Exposition. Sitzungsber. Wein Akad., 100: 704--729. Kimball, H. H., 1919. Variations in the total and luminous solar radiation with geographical position on horizontal, vertical and sloping surfaces. Mon. Weather Rev., 47: 615--629. Lee, R., 1964. Potential insolation as a topoclimatic characteristic of drainage basins. Bull. Int. Assoc. Sci. Hydrol., 9: 27--41. Morris, R. E. and Barry, R. G., 1963. Soil and air temperature in a New Forest valley. Weather, 18: 325--331. Parker, J., 1952. Environment and forest distribution of the Palouse Range in northern Idaho. Ecology, 33: 451--461. Potzger, J. E., 1939. Microclimate and a notable case of its influence on a ridge in central Indiana. Ecology, 20: 29--37. Reid, I., 1972. The Geomorphological Significance of Certain Soil Water Behaviour. Unpubl. Ph.D. Thesis, University of Hull. Reid, I., 1973a. The influence of slope orientation upon the soil moisture regime, and its hydrogeomorphological significance. J. Hydrol., 19: 309--321. Reid, I., 1973b. The influence of slope aspect on precipitation receipt. Weather, 28: 490--494. Reid, I., 1975. Seasonal variability of rainwater redistribution by field soils. J. Hydrol., 25: 71--80. Sakanoue, T., 1959. Microclimatic investigation of a waste heap. J. Agric. Meteorol. (Japan), 15: 59--63. Shanks, R. E. and Norris, F. H., 1950. Microclimatic variation in a small valley in eastern Tennessee. Ecology, 31: 532--539. Shreve, F., 1924. Influence of slope exposure on soil temperature. Carnegie Inst. Yearb., 23: 140--141. Smith, A., 1929. Diurnal, average and seasonal soil temperature changes at Davis, California. Soil Sci., 28: 457--468. Swift, L. W. and Van Bavel, C. H. M., 1961. Mountain topography and solar energy available for evapotranspiration. J. Geophys. Res., 66: 2565. Swift, L. W., 1976. Algorithm for solar radiation on mountain slopes. Water Resour. Res., 12: 108--112. Taylor, J. A., 1958. Growing season as affected by land aspect and soil texture. Univ. College of Wales at Aberystwyth Memo., 1, 33 pp. Watanabe, Y., 1959. The late sowing culture of wheat on the southern inclined ridges. J. Agric. Meteorol. (Jap.), 15: 15--18, Wolfe, F. N., Wareham, R. T. and Scofield, H. T., 1943. The microclimates of a small valley in central Ohio. Trans. Am. Geophys. Union, 154--166. Wollny, E., 1878. Untersuchungen fiber den Einflus der Exposition auf der Erw~irmung des Bodens. Forsch. Geb. Agrik. Phys., 1 : 263--294.
SOIL ENVIRONMENT AND THE HYDROMETEOROLOGICAL MOSAIC*
IAN REID
Birkbeck College, University of London, London (Great Britain) (Received March 9, 1977; accepted May 10, 1977)
ABSTRACT Reid, I., 1977. Soil environment and the hydrometeorological mosaic. Agric. Meteorol., 18: 425--433. Seasonal changes in the physical status of the soil are largely a response to progressive changes in magnitude and balance of hydrometeorological variables. Soil temperature may, other things being equal, show a comparatively simple response, the spatial inhomogeneity resulting from differences in ground slope and aspect. In the case of resident water, the soil exerts its own set of limiting conditions. As a result, the soil moisture pattern need not reflect the hydrometeorological mosaic. The results of a year's experiments at Caydell, North York Moors, England, are presented. INTRODUCTION
Seasonal adjustment in the magnitude and mutual balance of the hydrometeorological variables should be reflected in a changing physical status of the soil. Of the control variables, solar radiation plays a dominant role (Davenport, 1967), but energy exchanges at the ground surface are determined in part by local slope. Since the form of the land is complex it follows that there will exist an hydrometeorological mosaic at least as complicated. This in itself merely defines a pattern of potential process-response and is rendered simple by ignoring the additional complications that attend vegetational diversity. Such a situation is the setting for the hydrometeorological experiments at Caydell, North York Moors. Within this small valley, observations show that response variables such as soil temperature demonstrate spatial inhomogeneities which approach those predicted by the potential model. Other response variables, principally soil water content and actual evapotranspiration, diverge significantly. The soil exerts its own set of limiting conditions. Because of this the resulting spatial pattern of soil moisture content need not conform to the hydrometeorological mosaic. * A pattern of contiguous land units, differentiated hydrometeorologically by ground slope and aspect.
426
This is particularly true at a time of the year when crop growth rates are critical. Prediction of subtle aspect-controlled differences in spring soil temperatures and the adjustment of field management will benefit germination and protect seedlings, especially where crops are sensitive or where species have been newly introduced to British farming. At a later stage, an ability to make short-term forecasts of changing summer soil conditions becomes vital to the rational deployment of artificial soil water recharge. Given increasing use of sprinkler irrigation in low rainfall areas and a growing awareness of limited water resource, greater precision of estimated water-needs is desirable on farms that cover a variety of slope aspects. The influence of ground slope and azimuth upon soil environmental response to hydrometeorological variables was first studied by Wollny (1878). Since then the influence of slope aspect has been developed by two main schools: the German micro meteorologists, which have included Kerner (1891), Held (1941), Grunow (1953), Heigel (1958), Hartmann et al. (1959), and, of course, Geiger (1966); and the American ecologists led by Shreve (1924), but including Gail (1921), Bates (1923}, Cottle (1932), Potzger (1939), Wolfe et al. (1943), Shanks and Norris (1950), Parker (1952), and Cantlon (1953). More recently, the Japanese have been represented by Sakanoue (1959), and Watanabe (1959), while a new wave of interest has been shown by North American hydrologists and micrometeorologists, (Swift and Van Bavel, 1961; Lee, 1964; Ferguson et al., 1971; Swift, 1976). The British literature has been less prolific, which may reflect a subconscious feeling that the equable airmass dominated weather of the British Isles dampens down any large aspectinduced inhomogeneities. Nevertheless, it includes contributions from Taylor (1958), Morris and Barry (1963), Curtis (1971}, and Reid (1973a). EXPERIMENTAL GROUND
Caydell is a west--east incision of the Hambleton Hills (Fig.l). Its southand north-facing sideslopes show considerable geomorphological symmetry. The valley drops 100 m from a surrounding plateau which provides an uncomplicated skyline. This simplifies the pattern of solar radiation receipt, and the influence of topography upon airflow. In short, there is a distinct break between plateau and valley environments. Sites were selected on opposing valley sideslopes undifferentiated by geological factors (the local dip is insignificant}, or by vegetation (both are grazed permanent pasture}. Rainfall, soil moisture, soil drainage, and soil temperature were measured at regular weekly intervals from October, 1969 to October, 1970. Rainfall was determined by a network of 1 autographic and 19 nonrecording gauges, soil moisture was recorded for 5 soil depths at 36 sites, soil temperature at 10, 20 and 30 cm at 4 sites and soil drainage in 3 banks of monolith lysimeters. Details of the sampling variance of soil water content, and of the spatial patterns of rainfall induced by slope aspect are given elsewhere (Reid 1973a, b). The thermometers were Meteorological Office pattern
427
I
-
/ /
0+54°
,:-~
Caydell
t
• ,
1
,.
, ",:
L
-..
~
~ "k~
' -
Hull
"
/
-.
0
_
-
.
. . . . . .
qL
- -152 3
J ',::.-:-~
-I
,
-
"',,
,
. ........................... Contours
in m e t r e s
above
-
L------Z"
Z --- . . . . . . .
°
]
?:"
O D
/
~-
Metres
0
500
t
J
Fig.1. Index maps of Caydell. Contours extracted from O.S. Sheet SE 58 NW.
bent-stem mercury-in-glass, each calibrated against a National Physics Laboratory certified thermometer. Soil temperatures are 09h00 GMT readings. The site readings axe averaged to give a value for a soil block between 10 cm and 30 cm depth. No record of the diurnal penetration of the heat-wave into the soil was attempted at Caydell, but other results (Keen and Russell, 1921; Smith, 1929; Blanc, 1958) suggest that the 09h00 reading corresponds to a daily minimum at these soil depths. SOIL TEMPERATURE
The thermal character of advected air-masses affects all slopes in a similar manner. Any differentials in soil temperature at sites of different aspect or ground slope are, therefore, largely a reflection of differences in receipt of short-wave radiation. This assumes that soil response (thermal conductivity, specific heat, and so on) at each site is similar. For Caydell, the important soil variables of porosity and organic matter are not significantly different between sites (Reid, 1972). A major factor in the soil heat budget, however, is the accumulated difference in soil water content that develops in summer between south- and north-facing slopes. Its effect upon soil temperature response will be referred to later. The horizon-shadow diagram (Fig.2) illustrates for selected dates the approximate position of maximal sunlight penetration into Caydell relative to the thermometer stations. The south-facing site is always clear of the
428
NST4
...7 .5
NST 2 NST4
f J
s--'-'-
J
1. 2. 3. 4. 5. 6. 7.
October October November December February February March
N
f
13 27 10 22 2 16 2
Fig.2. Mid-day horizon-shadow, valley slope profiles, and soil thermometer sites.
horizon-shadow. NST3 and NST2 receive no direct sunlight for 18 weeks (midOctober to early March) while NST4, further downslope on the north-facing side of the valley, is hidden for approximately 10 weeks. The influence upon soil temperature of increasing time spent in shadow is reflected in a ranking of the intersite correlation coefficients. The south-facing site (SST1) correlates best with NST4, followed by NST3 and NST2. Ranking of coefficients involving NST2 produces an order exactly opposite. At this gross level of analysis, soil temperature follows an expected spatial pattern and is a reasonably predictable response variable of the hydrometeorological mosaic. This is further attested by plotting differences in temperature between aspect as a function of time (Fig.3). There is a progressive increase in differential warming of Caydell slopes in late winter and spring. During the sampling period, this is interrupted in January by a reduction in sunshine associated with an influx of comparatively
429 A. Sunshine and Air Temperature
4°I IINHIIIIHHIIWIm ~o I
~8~
~ 1 ";2o
-5.1
B.
Soil
Temperature
2'0 y = 1.9437,0.3267× 1"5
• 0-0115×2 • 0.0001× :~
r = 0'596
U ,L_.
•
•
p~
05 0"0
,
,
Nov.
D e c . Jan
,
•
Feb
M a r , Apr
May
Jun.
,
dul.
Aug.
•
,
,
Sept. Oct.
Fig.3. D i f f e r e n c e s in soil t e m p e r a t u r e a t SST1 (Ts) a n d N S T 3 (Tn), a n d s u n s h i n e h o u r s (n) a n d air t e m p e r a t u r e (Ta).
warm air and attendant cloud cover. Yet the rise in differences towards a maximum at the spring equinox is not halted. By the beginning of March the north-facing slope has emerged from its winter shade and from then until the solstice comes increasingly under the influence of direct sunlight. Soil temperature differences fall correspondingly. Following the solstice, the slightly moister soil of the north-facing slope, giving greater thermal capacity and perhaps lower thermal diffusivity, ensures that differences in soil temperature remain low even though the sun is now sinking. But differences inevitably increase as the north-facing slope moves towards its winter shade once more. Residual warmth is responsible for the lag in the development of maximum differences between slope in autumn. The north-facing slope soil cools as radiation inputs get smaller, while the south-facing slope soil is merely allowed to cool less rapidly. This difference in behaviour between a u t u m n and spring is emphasized by comparing the out-of-phase nature of equinox and m a x i m u m soil temperature differences in autumn with their coincidence on the warming limb of the annual cycle. The result is an a s y m m e t r y in the general biannual pattern of rise and fall (Fig.3). The limbs of the spring fluctuation are longer than are those of the autumn rise and fall, yet the maximum differences achieved are similar. SOIL WATER
Inequalities in soil water content do not reflect so simply the potential
430
spatial pattern which might derive from differences in solar radiation receipt alone. This is partly a function of the delicate and changing balance of the water-holding and -handling capacities of the soil, and partly due to the differential regulation of actual evapotranspiration under conditions of increasing soil water deficit. Broadly, the pattern is as expected (Fig.4) with greatest differences between aspect occurring in summer. To facilitate direct comparison, current water contents are expressed in ratio with the mean winter soil moisture content since it can be demonstrated (Reid, 1975) that this value is a sensitive indicator of the water-holding and -handling capacities of the soil. Having derived these values for all soil depths and all sampling weeks, the south-facing is subtracted from the north-facing to construct Fig.4. From mid-November until the end of April, the soil water content moves around the winter mean values of both north- and south-facing slopes. There is little hydrometeorologically induced spatial inequality, though if precipitation inputs decrease, as in March, the south-facing slope tends to slight relative dryness. The onset of drought in May terminates significant replenishment of the soil water reservoir which until now has been feeding the evapotranspiration process at rates unhindered by soil moisture deficit. At the same time, potential evaporative demand continues to increase as the sun moves towards the summer solstice. This demand is at all times significantly higher for the southfacing slope. Progression of a drying front into the soil of the south-facing slope initially promotes large differences in soil water content between aspect. However, from the beginning of June, two factors combine to reduce the inequality that has developed. The exaggerated soil moisture deficit of the south-facing slope maintains actual evapotranspiration considerably below the local potential demand. The south-facing slope soil remains, therefore, moister than would be expected. The second factor is that the north-facing slope, having initially suffered smaller water losses at the onset of drought, continues to lose water by evapotranspiration at rates which eventually exceed those of the south-facing slope (Reid, 1973a). Summer spatial inequalities in soil moisture are not of a magnitude expected from the mosaic of hydrometeorological demands. In general, soil--plant systems on south-facing slopes tend to conserve moisture. CONCLUSIONS
Theoretical modelling of control variables in the hydrometeorological mosaic is feasible (Kimball, 1919). Computation of the changing magnitude of actual solar radiation dose for any given slope has been facilitated by the development of suitable algorithms (Swift, 1976), and relies upon the adequacy of available data. More problematical is the modelling of the mosaic response variables. Caydell demonstrates varying degrees of predictability. Soil temperature response is comparatively simple. On the other hand, changing
431 "0
m m
9 0 u
-~
m
q
I .,..=. i
z m u r
n
o
m
m u
m ~ m
m- Z
lww) Fte~u!eu
{ ~ 3 ) qldeO I~0S
0
432
soil water content relies upon the variability in the rates of such processes as actual evapotranspiration which are in part regulated by soil moisture deficit, a negative feedback. To the British farmer, the small aspect controlled differences in minimum soil temperature during spring and autumn determine frost susceptibility. All except two observations of frozen-ground at Caydell indicate comparatively favourable conditions on the south-facing slope. But the phenological advantage gained in spring through higher soil temperatures on south-facing slopes may be more than compensated by the earlier onset of soil drought in early summer.
ACKNOWLEDGEMENTS
The programme of field experiments was carried out while sponsored by the Natural Environment Research Council. Thanks are due to Mr. P. Guthrie who owns a substantial portion of Caydell, and to the Headmaster, Ampleforth School for allowing access to the daily records of their meteorological station.
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