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Water use by tree plantations
Journal
0/ Hydrology 5 (1967) 206-213; ©
North-Holland Publishing Co., Am sterdam
Not to be reproduced by photoprint or microfilm without written permission from the publisher
WATER USE BY TREE PLANTATIONS R . KITCHING Forestry Commission , Alice Holt Re search St ation, Farnham, Surrey, U.K."
Abstract: The water balances of tree plantations have been investigated at three sites during 1965. Soil moisture has been assessed using the neutron probe technique. Percolation has been estimated during the early winter when absolute errors in estimates of evaporation are likely to be low. The mean value obtained for the ratio of evaporation of pines to open water was 0.7 ± 0.25.
Introduction
In comparison with grass and agricultural crops, few results have been published dealing with the water balance of trees. This has probably been in some part because of the difficulty of measuring the components of the water balance for trees - it is not very easy to construct a lysimeter of adequate dimensions to contain trees under natural conditions comparable with the surrounding area. Some work has been reported dealing with catchment areas. However, it is not always possible to be sure that a catchment is watertight and more particularly, catchments are not easily amenable to the comparison of different tree species. Law-) has reported some very high values for the evaporation of Sitka spruce in a large lysimeter, but this work has been criticised because of the inadequate surround to the trees. Rutter s) has reported some figures for the transpiration of Scots pine using a technique with gypsum resistance blocks. The occasions of departure from and return to field capacity are recognised using the resistance blocks, and moisture contents of the soil are assumed to be identical on these occasions. Measurement of the rainfall and estimation of the capillary rise enables the evaporation of the pine to be deduced between the two dates mentioned. The difficulties in this technique are (i) that the period of water use is beyond the control of the experimenter, so that quantitative comparisons between species and types of vegetation are impossible over the same period; (ii) resistance blocks are least sensitive at the field capacity portion of the range , they suffer hysteresis and their calibration tends to drift. .. The Author's present address is Water Department, Institute of Geological Sciences, London, S.W. 7.
206
WATER USE BY TREE PLANTATIONS
207
The aim of the present work is to deduce the evaporation of the trees by measuring the other components of the water balance. It was also considered desirable to develop a method which can be used to study water loss from trees over short periods of time. Such information would be extremely useful in investigations of the Forestry Commission concerning moisture stress in forestry plantations. Consequently it was considered important to be able to define the moisture status of a tree crop at any period in time. This involves an accurate measurement of the soil moisture content in the rooting zone. For this purpose, resistance blocks were rejected because of the considerable difficulties and uncertainties of calibration and hysteresis. Gravimetric sampling of the soil was attempted but proved extremely laborious and destructive of the site. It was concluded that the most promising method to yield the results required was the neutron scattering technique, and this paper deals with the results obtained during the first year of use of this equipment. The chief advantages of the neutron scattering method are as follows: I) Minimum disturbance to the site-backfilling of large installation holes is not required. 2) Repeated measurements can be made at the same site. 3) Accurate moisture content measurements can be made at any time of year. 4) The method measures a large sample. 5) The instrument is equally sensitive over the range of moisture contents met with in this work. Experimental details
Observations have been made in 2 experimental areas at each of the three sites listed in Table I. Further notes to Table I I) Bracken was present under the Corsican pine at Bramshill. 2) The Sitka spruce at Burley had suffered defoliation by Neomyzaphis in recent years. Also some trees had been affected by Armillaria. 3) There was no evidence of surface run-off at any site. 4) In all cases, the experimental sites were located in areas of extensive forest of similar height. The soil moisture measurements were made with a Nuclear Enterprises Neutron Probe and scaler. In each experimental area five neutron probe access tubes were installed in random positions. The following technique was used to install the access tubes for the neutron probe: A soil auger is used inside an open ended stainless steel cutting tube pressed vertically a short distance into the soil. Alternate au gering and
208
R. KITCHING
TABLE 1
Site
Elvetham, Bramshill Forest, Hants. Rendlesham, Aldewood Forest, Suffolk. Burley, New Forest, Hants.
Species
Planted
c.r.
c 1924
25
>3
2. D.F.
c 1924
25
5
1. C.P.
1920
20
>5
2. D.F.
1926
20
1. C.P./ S.P.Mix
1945
11
2. S.S.
1944
11
Soil
Deep Freely Drained Sandy Loam Deep Freely Drained Sand Deep Freely Drained Sand
Water Table mBelow Surface
Height (m)
1.
Variable > 2.5
pressing is done until the required depth is reached. It has been found helpful to use a sequence of augers, and tubes are one, two and three metres in length to avoid working at an inconvenient height above the ground. On removal of the cutting tube the hole is inspected carefully from the surface using a torch. If any cavities exist in the walls of the hole, due to stones being dislodged, the soil is repacked carefully in sequence into the hole. The auger and tube are then used as before, which yields a smooth-walled hole, and since little of the repacked soil remains in the hole the disturbance is minimal. Finally an aluminium alloy tube of appropriate length, 4.45 em diameter, with a plugged lower end, is packed carefully into the hole and closed with a rubber bung under which hangs a bag of silica gel. The bung is flush with the soil and covered over with litter. Soil moisture measurements were made at approximately fortnightly intervals at each site. Measurements were made at 10 em increments down each access tube except at the surface where a single measurement was made to assess the soil moisture in the top 12 em using a special calibration. The duration of each measurement was one minute. One access tube at each site selected at random was used to provide a calibration curve for the equipment. Undisturbed soil samples were taken close to an access tube with a sampling tube of approximate diameter 5 em. The samples were stored in polythene bags, weighed and oven dried in the laboratory. At Bramshill the rainfall was measured weekly at a site clear of trees and fairly close to the experimental area. The rainfall was also measured daily at
209
WATER USE BY TREE PLANTAnONS
Hartley Wintney Sewage Farm about 2 km away. At Rendlesham the rainfall was measured daily at the forest office about I km away and also at Butley about I km away. At Burley the rainfall was measured daily at the forest office (I km). At each site some pairs of tensiometers were installed at depths of 170 em and 240 em to indicate the magnitude of the moisture tension gradient below the tree rooting zone. Results
Fig. I shows the calibration graph obtained for the neutron probe. In spite of the method of sampling, the calibration points fall reasonably close to the NEUTRON
PROBE
CALIBRATION
30
o
25
10
5
O~'----'-------'----'--_-L_---'----'
o
2
8
6
10
c. p.m.
SLOW
NEUTRON
Fig. 1.
COUNT
RATE
210
R. KITCHING
line of slope 312.5 cpm/ % vol water. Absence of any appreciable amounts of neutron absorbers and organic matter below the surface layers is thought to be the cause of this result. It is to be expected that in other soils the calibration graph would be different. A separate calibration was constructed for the surface 12 cm where the effects of organic matter and loss of neutrons from the soil would occur. This was done by assuming the surface layer to have a uniform moisture content indicated by the gravimetric sampling and constructing an actual count/corrected count graph. This corrected count could then be compared with other standard counts when estimating changes of storage in the profile. For each access tube the count rates were totalled to a depth of 180 em at approximately the beginning and end of the growing season of the trees. The difference between the totals over the growing season was noted for each tube. The mean of these differences for each site was then converted to em of water using the slope of the calibration graph. This gave the change in soil moisture storage over the growing season, an increase to be considered positive, a decrease negative. The most difficult item of the water balance to estimate was the loss of water from the profile by vertical drainage. This loss has often been neglected or equated to zero in previous investigations. It is however, important to estimate this quantity and its probable error in order that the effect upon any estimate of evaporation may be ascertained. An attempt has been made to estimate the vertical drainage component in this work. The principle that has been used is that of evaluating the water balance during a period in the winter when the evaporation is low. If for this period an estimate of evaporation is inserted into the water balance equation, the only unknown quantity is the vertical drainage. An error in the estimate of evaporation will have a small effect upon the calculated drainage at this time of year. It is necessary to choose an appropriate period of the early winter when the wetting front has not penetrated far down the profile and the drainage may be expected to be of the same order as during the summer season. Such periods have been selected for all the sites on the basis of the neutron probe and tensiometer readings. The daily vertical drainage data are shown in Table 2. It was also possible to obtain a rough check upon the magnitude of the vertical drainage during the summer period by using the tensiometer data and applying Darcy's Law with values of conductivities obtained by other workers for similar soils 3 • 4 ) . The results of this calculation are shown in Table 3. Fair agreement is obtained between the results in Table 2 and those in Table 3. However, it should be borne in mind that the tensiometer data used was somewhat limited and the use of tensiometers at such depths in
211
WATER USE BY T REE P LANT ATIO NS
T ABLE 2
Bramshill
c .e. Bramsh ill D .F. Ren dlesham
c .r . Rendl esham D .F . Burley CP/S P Burley S.S.
Estim ated
D a ily Flow D o wn em /day
Ra infall em ± 5%
So il Moist ure Increase em
± 50 % cm
em
29/9 ->- 24/ 1 118 da ys I /I 0 ->- 26/ 11 57 da ys 23/9 - >4/1 1 43 da ys 23/9->- 4/ 11 43 days
2104
+ 8.85 ± 0.77
3.48
9. 1
0.077
7. 15
+ 1.83 ± 0.94
2.21
3. 1
0.054
4.98
- 1048 ± 0.32
3.12
3.3
0.077
4.98
- I. 75 -': 0042
3.12
3.6
0.084
6/1 0~> 24/1 1
8.56
!
1.76 . 0.22
2.19
4.6
0.0 91
50 days 6/ 10 24/11 50 da ys
8.56
i 0.23
0.2 7
2.19
6. 1
0.1 22
Per iod 1965
Locati on / Species
R esidu al
ET
TAB LE 3
Locat ion
Bramshill
Mean T. 96 em I-1 g
Mean T.66 em Hg
15.3
2 1.0
7.0
3.0
0.008 3 )
0.33
6.6
7. 1
0.004 4 )
0.087
4.7
3.9
0.02 4 )
0.55
1504
11.8
0.001 4 )
0.039
8.9
8. 1
0.01 4 )
0.274
K cm /h
Fl o w em/d ay
0
c.r. Bram sh ill D.F. Rendlesharn
c.r. RendJesh am D .F . Burley CP/SP Burley S.S.
such soils gives rise to unce rt aintie s beca use of the impossibility of install ing them without di st urb ing the soil. It is also que stionabl e whether co nd uctivities from other soil s may be read ily a pplied to thi s work without introducing error. For the se reasons the vertical dr ainage data in Table 2 are considered to be m ore reliable and have been used in computing the wate r balance s in Table 4.
212
R. KITelliNG
TABLE
4
-----
Location
Bramshill C.P. Brarnshill D.E Rendlesham C.P. Rendlesham D.E Burley CP/SP Burley S.S.
Period 1965
11/5-*29/9 141 days 10/5 -* 1/10 144 days 18/5 -*23/9 128 days 18/5 -*23/9 128 days 21/5 -* 22/9 124 days 21/5 -*22/9 124 days
Rainfall Soil Moisture em Increase em ±5%
Drainage
ET
Eo
em
cm
em
30.4 -1.87 ± 1.60
10.8 ± 5.2
21.5 38.2
0.57 ±0.23
31.1 -1.23 ± 1.07
7.8 ± 6.5
24.538.9
0.63 ±0.24
28.1 -4.28 ±0.72
9.8 ±6.8
22.632.1
0.71 ±0.28
28.1 +0.04±0.12
1O.8±7.5
17.3 32.1
0.54±0.28
33.1 - 8.15 ± 1.11
11.3 ±4.7
30.036.95 0.81 ±0.23
33.1 -0.02±0.65
15.1 ±4.5
18.036.95 0.49±0.18
ET/Eo
Table 4 shows the components of the water balance at the sites for the summer period of 1965. The soil moisture increase and vertical drainage are computed as described above. The derived evaporation at each site for the period stated is listed in column E T • In order to compare the results of this work with some previous work it is convenient to compare E T with Eo, the theoretical evaporation for an open water surface as calculated by Penman's equations) from local meteorological data. The ratio ET/Eo for each site is shown in the final column of Table 4. It is necessary to consider how errors may have arisen in the derivation of the estimates of ET/Eo. Small errors may have occurred in the estimates of precipitation. A check rain gauge was read weekly in a clear area near the experimental sites at Bramshill. The catch was usually about + 5 % higher than the gauge at Hartley Wintney but its exposure was not ideal. The Hartley Wintney figures are considered to be more reliable although this rain gauge was further from the experimental site. At Rendlesham, rainfall was obtained from two stations close by which differed usually by about 5 %. The mean reading of the two gauges was used in these calculations. Only one rain gauge was installed near the Burley site. Thus the error due to the measurement of rainfall over the whole period was probably not greater than 5%. The standard error in the difference of totals of soil moisture was usually less than 1.5 em over the summer season for one species at one site. This was calculated using the slope of the line in Fig. 1. An error of 10% in the slope of the calibration curve would yield a maximum error of about 1 em in the
WATER USE BY TREE PLANTATIONS
213
case of the Corsican/Scots pine mixture at Burley. Thus the maximum likely error in the soil moisture component was approximately 2.5 em. The largest source contributing to the error in E T was probably the estimation of the vertical drainage. Since this was computed on a water balance basis it is subject to the errors listed above and, additionally, to the possible error in the estimated E T during the winter season. Considering that the winter evaporation from trees is unlikely to exceed by more than 50 % the average evaporation from other vegetation and adding in the other errors, the total error in the ratio E T / Eo is given in the last column of Table 4. These errors are likely to be overestimated because of the generous allowance of 50 % on the winter estimate of E T when computing the vertical drainage. Even with these errors the values obtained from the transpiration of these trees are considerably less than the values obtained by Rutter using the gypsum block technique. it is possible that the gypsum block technique yields errors because of more intense rooting in the volume of disturbed soil in the region of the block. On the only occasion when a direct comparison was made between soil moisture sampling and tensiometer methods 6) a lower result was obtained for the soil moisture sampling. The neutron scatter technique samples a relatively large volume of undisturbed soil and would be less affected by any small disturbances in the region of the access tube. it should also be noted that Rutter dealt with Scots pine while the present work deals in part with Corsican pine. Note that the highest value of evaporation obtained in this work was at Burley where the Corsican pine was mixed with Scots pine. References I) 2) 3) 4) 5) 6)
F. LAW, International Association of Scientific Hydrology 44 (1957) 397 A. J. RUTTER, J. Appl. Ecol. 1 (1964) 29 W. GARDNER, Soil Sci. 93 (1962) 271 L. A. RICHARDS, Trans. Am. Geophys. Union 33 (1952) 531 H. L. PENMAN, Netherl. J. Agric. Sci. 1 (1956) 9 A. J. RUTTER, Symposium of Hannoversch-Munden 1 (1959) 101
0/ Hydrology 5 (1967) 206-213; ©
North-Holland Publishing Co., Am sterdam
Not to be reproduced by photoprint or microfilm without written permission from the publisher
WATER USE BY TREE PLANTATIONS R . KITCHING Forestry Commission , Alice Holt Re search St ation, Farnham, Surrey, U.K."
Abstract: The water balances of tree plantations have been investigated at three sites during 1965. Soil moisture has been assessed using the neutron probe technique. Percolation has been estimated during the early winter when absolute errors in estimates of evaporation are likely to be low. The mean value obtained for the ratio of evaporation of pines to open water was 0.7 ± 0.25.
Introduction
In comparison with grass and agricultural crops, few results have been published dealing with the water balance of trees. This has probably been in some part because of the difficulty of measuring the components of the water balance for trees - it is not very easy to construct a lysimeter of adequate dimensions to contain trees under natural conditions comparable with the surrounding area. Some work has been reported dealing with catchment areas. However, it is not always possible to be sure that a catchment is watertight and more particularly, catchments are not easily amenable to the comparison of different tree species. Law-) has reported some very high values for the evaporation of Sitka spruce in a large lysimeter, but this work has been criticised because of the inadequate surround to the trees. Rutter s) has reported some figures for the transpiration of Scots pine using a technique with gypsum resistance blocks. The occasions of departure from and return to field capacity are recognised using the resistance blocks, and moisture contents of the soil are assumed to be identical on these occasions. Measurement of the rainfall and estimation of the capillary rise enables the evaporation of the pine to be deduced between the two dates mentioned. The difficulties in this technique are (i) that the period of water use is beyond the control of the experimenter, so that quantitative comparisons between species and types of vegetation are impossible over the same period; (ii) resistance blocks are least sensitive at the field capacity portion of the range , they suffer hysteresis and their calibration tends to drift. .. The Author's present address is Water Department, Institute of Geological Sciences, London, S.W. 7.
206
WATER USE BY TREE PLANTATIONS
207
The aim of the present work is to deduce the evaporation of the trees by measuring the other components of the water balance. It was also considered desirable to develop a method which can be used to study water loss from trees over short periods of time. Such information would be extremely useful in investigations of the Forestry Commission concerning moisture stress in forestry plantations. Consequently it was considered important to be able to define the moisture status of a tree crop at any period in time. This involves an accurate measurement of the soil moisture content in the rooting zone. For this purpose, resistance blocks were rejected because of the considerable difficulties and uncertainties of calibration and hysteresis. Gravimetric sampling of the soil was attempted but proved extremely laborious and destructive of the site. It was concluded that the most promising method to yield the results required was the neutron scattering technique, and this paper deals with the results obtained during the first year of use of this equipment. The chief advantages of the neutron scattering method are as follows: I) Minimum disturbance to the site-backfilling of large installation holes is not required. 2) Repeated measurements can be made at the same site. 3) Accurate moisture content measurements can be made at any time of year. 4) The method measures a large sample. 5) The instrument is equally sensitive over the range of moisture contents met with in this work. Experimental details
Observations have been made in 2 experimental areas at each of the three sites listed in Table I. Further notes to Table I I) Bracken was present under the Corsican pine at Bramshill. 2) The Sitka spruce at Burley had suffered defoliation by Neomyzaphis in recent years. Also some trees had been affected by Armillaria. 3) There was no evidence of surface run-off at any site. 4) In all cases, the experimental sites were located in areas of extensive forest of similar height. The soil moisture measurements were made with a Nuclear Enterprises Neutron Probe and scaler. In each experimental area five neutron probe access tubes were installed in random positions. The following technique was used to install the access tubes for the neutron probe: A soil auger is used inside an open ended stainless steel cutting tube pressed vertically a short distance into the soil. Alternate au gering and
208
R. KITCHING
TABLE 1
Site
Elvetham, Bramshill Forest, Hants. Rendlesham, Aldewood Forest, Suffolk. Burley, New Forest, Hants.
Species
Planted
c.r.
c 1924
25
>3
2. D.F.
c 1924
25
5
1. C.P.
1920
20
>5
2. D.F.
1926
20
1. C.P./ S.P.Mix
1945
11
2. S.S.
1944
11
Soil
Deep Freely Drained Sandy Loam Deep Freely Drained Sand Deep Freely Drained Sand
Water Table mBelow Surface
Height (m)
1.
Variable > 2.5
pressing is done until the required depth is reached. It has been found helpful to use a sequence of augers, and tubes are one, two and three metres in length to avoid working at an inconvenient height above the ground. On removal of the cutting tube the hole is inspected carefully from the surface using a torch. If any cavities exist in the walls of the hole, due to stones being dislodged, the soil is repacked carefully in sequence into the hole. The auger and tube are then used as before, which yields a smooth-walled hole, and since little of the repacked soil remains in the hole the disturbance is minimal. Finally an aluminium alloy tube of appropriate length, 4.45 em diameter, with a plugged lower end, is packed carefully into the hole and closed with a rubber bung under which hangs a bag of silica gel. The bung is flush with the soil and covered over with litter. Soil moisture measurements were made at approximately fortnightly intervals at each site. Measurements were made at 10 em increments down each access tube except at the surface where a single measurement was made to assess the soil moisture in the top 12 em using a special calibration. The duration of each measurement was one minute. One access tube at each site selected at random was used to provide a calibration curve for the equipment. Undisturbed soil samples were taken close to an access tube with a sampling tube of approximate diameter 5 em. The samples were stored in polythene bags, weighed and oven dried in the laboratory. At Bramshill the rainfall was measured weekly at a site clear of trees and fairly close to the experimental area. The rainfall was also measured daily at
209
WATER USE BY TREE PLANTAnONS
Hartley Wintney Sewage Farm about 2 km away. At Rendlesham the rainfall was measured daily at the forest office about I km away and also at Butley about I km away. At Burley the rainfall was measured daily at the forest office (I km). At each site some pairs of tensiometers were installed at depths of 170 em and 240 em to indicate the magnitude of the moisture tension gradient below the tree rooting zone. Results
Fig. I shows the calibration graph obtained for the neutron probe. In spite of the method of sampling, the calibration points fall reasonably close to the NEUTRON
PROBE
CALIBRATION
30
o
25
10
5
O~'----'-------'----'--_-L_---'----'
o
2
8
6
10
c. p.m.
SLOW
NEUTRON
Fig. 1.
COUNT
RATE
210
R. KITCHING
line of slope 312.5 cpm/ % vol water. Absence of any appreciable amounts of neutron absorbers and organic matter below the surface layers is thought to be the cause of this result. It is to be expected that in other soils the calibration graph would be different. A separate calibration was constructed for the surface 12 cm where the effects of organic matter and loss of neutrons from the soil would occur. This was done by assuming the surface layer to have a uniform moisture content indicated by the gravimetric sampling and constructing an actual count/corrected count graph. This corrected count could then be compared with other standard counts when estimating changes of storage in the profile. For each access tube the count rates were totalled to a depth of 180 em at approximately the beginning and end of the growing season of the trees. The difference between the totals over the growing season was noted for each tube. The mean of these differences for each site was then converted to em of water using the slope of the calibration graph. This gave the change in soil moisture storage over the growing season, an increase to be considered positive, a decrease negative. The most difficult item of the water balance to estimate was the loss of water from the profile by vertical drainage. This loss has often been neglected or equated to zero in previous investigations. It is however, important to estimate this quantity and its probable error in order that the effect upon any estimate of evaporation may be ascertained. An attempt has been made to estimate the vertical drainage component in this work. The principle that has been used is that of evaluating the water balance during a period in the winter when the evaporation is low. If for this period an estimate of evaporation is inserted into the water balance equation, the only unknown quantity is the vertical drainage. An error in the estimate of evaporation will have a small effect upon the calculated drainage at this time of year. It is necessary to choose an appropriate period of the early winter when the wetting front has not penetrated far down the profile and the drainage may be expected to be of the same order as during the summer season. Such periods have been selected for all the sites on the basis of the neutron probe and tensiometer readings. The daily vertical drainage data are shown in Table 2. It was also possible to obtain a rough check upon the magnitude of the vertical drainage during the summer period by using the tensiometer data and applying Darcy's Law with values of conductivities obtained by other workers for similar soils 3 • 4 ) . The results of this calculation are shown in Table 3. Fair agreement is obtained between the results in Table 2 and those in Table 3. However, it should be borne in mind that the tensiometer data used was somewhat limited and the use of tensiometers at such depths in
211
WATER USE BY T REE P LANT ATIO NS
T ABLE 2
Bramshill
c .e. Bramsh ill D .F. Ren dlesham
c .r . Rendl esham D .F . Burley CP/S P Burley S.S.
Estim ated
D a ily Flow D o wn em /day
Ra infall em ± 5%
So il Moist ure Increase em
± 50 % cm
em
29/9 ->- 24/ 1 118 da ys I /I 0 ->- 26/ 11 57 da ys 23/9 - >4/1 1 43 da ys 23/9->- 4/ 11 43 days
2104
+ 8.85 ± 0.77
3.48
9. 1
0.077
7. 15
+ 1.83 ± 0.94
2.21
3. 1
0.054
4.98
- 1048 ± 0.32
3.12
3.3
0.077
4.98
- I. 75 -': 0042
3.12
3.6
0.084
6/1 0~> 24/1 1
8.56
!
1.76 . 0.22
2.19
4.6
0.0 91
50 days 6/ 10 24/11 50 da ys
8.56
i 0.23
0.2 7
2.19
6. 1
0.1 22
Per iod 1965
Locati on / Species
R esidu al
ET
TAB LE 3
Locat ion
Bramshill
Mean T. 96 em I-1 g
Mean T.66 em Hg
15.3
2 1.0
7.0
3.0
0.008 3 )
0.33
6.6
7. 1
0.004 4 )
0.087
4.7
3.9
0.02 4 )
0.55
1504
11.8
0.001 4 )
0.039
8.9
8. 1
0.01 4 )
0.274
K cm /h
Fl o w em/d ay
0
c.r. Bram sh ill D.F. Rendlesharn
c.r. RendJesh am D .F . Burley CP/SP Burley S.S.
such soils gives rise to unce rt aintie s beca use of the impossibility of install ing them without di st urb ing the soil. It is also que stionabl e whether co nd uctivities from other soil s may be read ily a pplied to thi s work without introducing error. For the se reasons the vertical dr ainage data in Table 2 are considered to be m ore reliable and have been used in computing the wate r balance s in Table 4.
212
R. KITelliNG
TABLE
4
-----
Location
Bramshill C.P. Brarnshill D.E Rendlesham C.P. Rendlesham D.E Burley CP/SP Burley S.S.
Period 1965
11/5-*29/9 141 days 10/5 -* 1/10 144 days 18/5 -*23/9 128 days 18/5 -*23/9 128 days 21/5 -* 22/9 124 days 21/5 -*22/9 124 days
Rainfall Soil Moisture em Increase em ±5%
Drainage
ET
Eo
em
cm
em
30.4 -1.87 ± 1.60
10.8 ± 5.2
21.5 38.2
0.57 ±0.23
31.1 -1.23 ± 1.07
7.8 ± 6.5
24.538.9
0.63 ±0.24
28.1 -4.28 ±0.72
9.8 ±6.8
22.632.1
0.71 ±0.28
28.1 +0.04±0.12
1O.8±7.5
17.3 32.1
0.54±0.28
33.1 - 8.15 ± 1.11
11.3 ±4.7
30.036.95 0.81 ±0.23
33.1 -0.02±0.65
15.1 ±4.5
18.036.95 0.49±0.18
ET/Eo
Table 4 shows the components of the water balance at the sites for the summer period of 1965. The soil moisture increase and vertical drainage are computed as described above. The derived evaporation at each site for the period stated is listed in column E T • In order to compare the results of this work with some previous work it is convenient to compare E T with Eo, the theoretical evaporation for an open water surface as calculated by Penman's equations) from local meteorological data. The ratio ET/Eo for each site is shown in the final column of Table 4. It is necessary to consider how errors may have arisen in the derivation of the estimates of ET/Eo. Small errors may have occurred in the estimates of precipitation. A check rain gauge was read weekly in a clear area near the experimental sites at Bramshill. The catch was usually about + 5 % higher than the gauge at Hartley Wintney but its exposure was not ideal. The Hartley Wintney figures are considered to be more reliable although this rain gauge was further from the experimental site. At Rendlesham, rainfall was obtained from two stations close by which differed usually by about 5 %. The mean reading of the two gauges was used in these calculations. Only one rain gauge was installed near the Burley site. Thus the error due to the measurement of rainfall over the whole period was probably not greater than 5%. The standard error in the difference of totals of soil moisture was usually less than 1.5 em over the summer season for one species at one site. This was calculated using the slope of the line in Fig. 1. An error of 10% in the slope of the calibration curve would yield a maximum error of about 1 em in the
WATER USE BY TREE PLANTATIONS
213
case of the Corsican/Scots pine mixture at Burley. Thus the maximum likely error in the soil moisture component was approximately 2.5 em. The largest source contributing to the error in E T was probably the estimation of the vertical drainage. Since this was computed on a water balance basis it is subject to the errors listed above and, additionally, to the possible error in the estimated E T during the winter season. Considering that the winter evaporation from trees is unlikely to exceed by more than 50 % the average evaporation from other vegetation and adding in the other errors, the total error in the ratio E T / Eo is given in the last column of Table 4. These errors are likely to be overestimated because of the generous allowance of 50 % on the winter estimate of E T when computing the vertical drainage. Even with these errors the values obtained from the transpiration of these trees are considerably less than the values obtained by Rutter using the gypsum block technique. it is possible that the gypsum block technique yields errors because of more intense rooting in the volume of disturbed soil in the region of the block. On the only occasion when a direct comparison was made between soil moisture sampling and tensiometer methods 6) a lower result was obtained for the soil moisture sampling. The neutron scatter technique samples a relatively large volume of undisturbed soil and would be less affected by any small disturbances in the region of the access tube. it should also be noted that Rutter dealt with Scots pine while the present work deals in part with Corsican pine. Note that the highest value of evaporation obtained in this work was at Burley where the Corsican pine was mixed with Scots pine. References I) 2) 3) 4) 5) 6)
F. LAW, International Association of Scientific Hydrology 44 (1957) 397 A. J. RUTTER, J. Appl. Ecol. 1 (1964) 29 W. GARDNER, Soil Sci. 93 (1962) 271 L. A. RICHARDS, Trans. Am. Geophys. Union 33 (1952) 531 H. L. PENMAN, Netherl. J. Agric. Sci. 1 (1956) 9 A. J. RUTTER, Symposium of Hannoversch-Munden 1 (1959) 101