Water balance estimates over Greece

Agricultural watermanagement ELSEVIER

Agricultural Water Management 32 (1996) 85-104

Water balance estimates over Greece P. Kerkides a,*, H. Michalopoulou b, G. Papaioannou b, R. Pollatou b a Laboratory of Agricultural Hydraulics, Agricultural 55 Athens b Laboratory of Meteorology, Department of Applied 80 Athens

University of Athens, 75 Iera Odos Str., Votanikos, I18 GR, Greece Physics, University of Athens, 33 Ippokratous Str., 106 GR, Greece

Accepted 14 March 1996

Abstract Water balance for 31 locations in Greece is calculated on the basis of long-term average monthly precipitation, evapotranspiration and combined soil and vegetation characteristics, according to the method proposed by Thomthwaite and Mather. Monthly evapotranspiration estimates are calculated from 27 years (1960- 1987) of routine meteorological data using the original Penman method. Soil and vegetation characteristics specific for the locations under study are combined in the water capacity of the root zone (WCRZ). Similar water balance calculations were carried out using various fixed values of WCRZ for all stations, to evaluate the effects of soil and vegetation through the WCRZ in the final estimates of soil moisture deficits. Water balance calculations were also performed using average monthly evapotranspiration estimates calculated according to the empirical Thomthwaite method. Results were compared in order to show possible differences that could be attributed to the method of estimating evapotranspiration. Finally, results obtained with a value of WCRZ fixed at 300 mm and potential evapotranspiration estimated by the Thomthwaite method for the period 1969-1987 were compared with existing similar results over a longer period in the past (1931-1968), in order to detect diachronic changes in the water balance components over the same regions in Greece. Keywords:

Water balance; Evapotranspiration; Soil moisture deficit

1. Introduction The term ‘water balance’ was introduced to refer to the balance between the input of water from precipitation and snowmelt and the outflow of water by evapotranspiration,

* Corresponding

author. Tel.: 00301-5294

0378-3774/96/$15.00 Copyright PI1 SO378-3774(96)01251-6

066; fax: 00301-5294

081

0 1996 Elsevier Science B.V. All rights reserved.

86

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32 (1996) 85-104

ground water recharge and streamflow. Among several possible methods of calculation of water balance, that introduced by Thomthwaite and Mather (1955) has been generally accepted. The method is fairly simple and uses long-term average monthly rainfall, long-term average monthly potential evapotranspiration and soil and vegetation characteristics. The last two factors are combined in the determination of water capacity of the root zone. Although the concept of water balance has been gradually evolved to more elaborate and sophisticated models (SWATRE, SWATRER, etc.; Belmans et al., 1983; Belmans, 1985) for restricted soil profiles, also taking into account vegetative cover and soil and crop specifications, Thomthwaite and Mather’s original model is still applicable in those parts of the world that are poorly monitored. Thus, for practical purposes the original model, improved by incorporating the Penman combination method for estimating potential evapotranspiration, appears to remain a very useful and relatively simple tool with which to obtain a countrywide picture of soil moisture, actual evapotranspiration, groundwater recharge and stream flow throughout the year. Interest in the water balance of Mediterranean countries can be traced back to Edmund Halley in 1687 (Monteith, 1981) and remains high since present-day water shortages are endemic throughout this region (Milliman et al., 1992). The whole area may be vulnerable to climate change (Kypris, 1993; Dragoni, 1993; Issar, 1993; Kypris, 1995; Palutikof et al., 19941, particularly through the effects on water balance and the implications for agricultural, domestic and industrial water supply. Thus, the present work has two basic objectives. First, to give a picture of the soil moisture availability or deficiency for the whole of Greece and at the same time to evaluate the effects of different evapotmnspiration estimates or soil and vegetation characteristics on the final estimates of soil water deficits. Second, to detect possible differences in the water balance through the years 193 l-1987. The second objective has been motivated from the published results of Karras (1973) covering the period between 193 1 and 1968 (with an unavoidable break between 1941 and 1948 due to the war); these results posed a challenge for further study. 2. Theory and methods 2.1. Water balance A recently published, detailed description of the method of calculation of ThomthWaite and Mather (1957) can be found in Dunne and Leopold (1978). A table showing the capacities of the root zone for several soil-crop combinations has also been included (table 8.2, p. 242). Here, the basic assumptions are restated and the pertinent water balance equations given for the sake of completeness. The following assumptions are made. 1. The soil profile may retain a maximum water height, the so-called water capacity of the root zone (WCRZ), beyond which moisture surplus is accumulated and deep percolation takes place. The water capacity of the root zone depends upon the texture of the soil (more specifically on its moisture retention curve> and the effective rooting depth, D, of the vegetation. This may be estimated by the expression WCRZ = D( 0,

- Cl,,,)

(1)

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Water Management 32 (1996) 85-104

or the average value of this when soil and vegetation whole catchment area WCRZ = CFiDi(

or,

- Or,,)

i

distribution

is considered

87

for the

(2)

is volumetric soil water content at field capacity, Or,, is volumetric soil where 0, water content at permanent wilting point and Fi is area1 percentage of vegetation with rooting depth Di. 2. When the soil profile retains water at its maximum capacity, evapotranspiration is taking place at its potential rate. 3. Actual evapotranspiration (AET) is considered as a decreasing function of the ratio AW/WCRZ, where AW denotes the actually available water retained in the soil profile. The estimation of AET, for the dry season (e.g. for the months with precipitation, P, less than potential evapotranspiration, PET, i.e. when P < PET) rests upon the empirical relationship (Thomthwaite and Mather, 1957; Dunne and Leopold, 1978) AW = WCRZ exp ( - APWL/WCRZ)

(3)

which gives AW as a function of the accumulated potential water loss (APWL). APWL is the cumulation of negative values of P - PET for the dry season only. It expresses the severity of the dry season, increasing during the sequence of months with PET > P. Summation begins at the end of the wet season. In other words, the soil exhibits a kind of resistance in the process of soil water withdrawal when soil moisture falls below field capacity. In this case AET is given by AET, = Pi + AWi_ 1 - AW,

4.

5.

6.

7.

(4)

where i indicates the sequence of months in the dry season. (For more details see Dunne and Leopold, 1978; Donker, 1987.) Soil moisture content throughout the soil profile is uniform and water leaves the soil profile in a step-wise fashion moving from right to left, i.e. from higher 0 values to lower ones. In an unsaturated soil profile where precipitation occurs during a considered time (monthly) interval At, but not in sufficient quantities to replenish the soil profile, it is assumed that this precipitation is consumed directly to meet the evapotranspiration demands before water is extracted from the reserves of the soil profile. The process of surface runoff (storm runoff or direct runoff), which depends on the intensity and the duration of the storm, the hydraulic characteristics of the upper soil layer and the relief, can be considered by subtracting a certain fraction of the precipitation. Values of this fraction may vary from 0 to 5% or more, depending on the area1 extent of the watershed and can be estimated from long-term average monthly direct runoff data. Drainage (deep percolation) is considered to take place when soil water is in excess of WCRZ and this is assumed to leave the soil profile as a certain fraction of the moisture surplus, the whole process being carried through successive monthly time intervals A r.

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P. Kerkides et al./Agricultural

Fig.

I. Schematic representation

Water Management 32 (1996) 85-104

of the water balance of a catchment

The water balance of a soil profile (or a watershed) natural conditions, can be expressed by P=I+R

(from Dune

and Leopold,

for a time period

1978).

At, under (5)

and I = AET + dSM + dGWS + GWR

(6) where P is precipitation, R is surface runoff, I is infiltration, AET is actual evapotranspiration, dSM is change in soil moisture of the unsaturated soil layer, dGWS is change of the water stored in the saturated soil profile and GWR is groundwater runoff. Fig. 1 gives schematically the various components entering the water balance equations. 2.2. Potential

evapotranspiration

When the required set of meteorological data is available for a local site, estimates of potential evapotranspiration, PET, are often calculated using a form of the combination equation. There is no longer much dispute over the relative superiority of the Penman method (Penman, 1948; Penman, 1963) of estimating PET in most practical situations. Furthermore, the use of even simpler formulae, based almost solely on air temperature, as the one given by Thornthwaite (1948) is approximate but useful for areas in which detailed meteorological records are lacking. Below, these two methods used in this study for calculating potential evapotranspiration are presented briefly. 2.3. Penman method The general form of the Penman PET,,,

= AR,/(

A + r)

equation

+ y&/C

is

A + r>

(7)

P. Kerkides

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32 (1996) 85-104

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is potential evapotranspiration (mm day- ’), A is the slope of the PET,, vapor pressure-temperature curve calculated at mean air temperature (hPa K-t), y is the psychometric constant (hPa K-l), R, is net long-wave and short-wave radiation (mm day-’ water equivalent) and E, is an aerodynamic vapor transport term (mm day- ’) of the form

where

saturation

E,=f(u)(e,-e,) (8) where J(U) is either an empirically or theoretically derived aerodynamic wind function, and (e, - e,) is the difference between saturated vapor pressure and actual vapor pressure (hPa), evaluated at mean air temperature and at 2 m above the ground or water surface.

Table 1 Characteristics of stations, spatial distribution of vegetation and soil texture of the corresponding eparchies and average estimates of the water capacity of the root zone (WCRZ) Station

Lat. (N)

35”OO’ 35”OO’ 35” 20’ 35” 30’ 36” 08’ 36” 24’ 36” 44’ 36” 50’ 37” 04 37” 06’ 37” 32’ 3P 54 37” 5.5’ 37” 58’ Pyrgos Argostoli 38” 11’ Chios 38” 21’ Aliartos 38” 22’ Agrinio 38” 37’ Lamia 38” 5 1’ Mytilini 39” 04 39” 22’ Voles Kerkyra 39” 27’ Trikala 39” 33’ 39” 39’ Larissa Ioannina 39” 42’ Trikala Hem. 40” 05’ Kozani 40” 17’ Mikra 40”31’ Alexan/lis 40” 5 1’ 41” 05’ Serres Komotini 41” 07’

Tymbaki Ierapetra Heraklion Chania Kythera Rhodes Miles Methoni Kalamata Naxos Tripoli Elliniko Adravida

Alt.

Spatial distribution

of vegetation

(m)

Cultiv. land

Pastures

Forests

6 31 39 62 167 11 183 52 31 10 652 15 12 132 22 4 110 25 17 5 2.6 4 110 74 484 100 626 5 3 34 61

62 40 67 33 12 17 13 50 25 22 26 4 54 54 20 18 43 22 26 50 38 55 27 67 12 46 25 59 26 43 37

29 40 19 57 64 35 70 15 42 63 48 3 12 12 66 64 42 43 39 19 43 23 45 23 56 21 55 23 40 32 21

1 16 1 2 19 36 1 27 24 2 22 17 24 24 10 11 8 22 28 26 15 5 19 3 26 23 10 3 28 9 34

(%)

Soil texture

geographical WCRZ

(mm) SC,SiC,C,Si,SCL,CL,SiCL Si,SCL,CL,SiCL Si,SCL,CL,SiCL SL,L,SiL Si,SCL,CL,SiCL Si,SCL,CL,SiCL SL,L,SiL L,SiL L,SiL,SL Si,SCL,CL,SiCL,SL,L,SiL SL SC,SiC,C SL,L,SiL S,LS Si,SCL,CL,SiCL Si,SCL,CL,SiCL SL,L,SiL L,CL Si,SCL,CL,SiCL SL,L,SiL SC,SiC,C,Si,SCL,CL,SiCL SC,SiC,C,Si,SCL,CL,SiCL CL,SiCL,SCL,Si SL,L,SiL Si,SCL,CL,SiCL SL,L,SiL Si,SCL.CL,SiCL Si,SCL,CL,SiCL SL,L,SiL Si,SCL,CL,SiCL SL,L,SiL

182 224 172 216 228 244 200 263 252 178 210 80 228 228 212 235 223 216 242 284 224 203 220 203 274 225 242 225 277 195 236

P. Kerkides et al./Agricultural

90

In the combination f(u)

equation,

Water Management 32 (1996) 85-104

the equivalent

to the wind function

(Penman,

1948) is

= 0.263( a, + b,u)

(9)

where u is wind speed (m s-l > at 2 m elevation, and a, and b, are empirical coefficients. Penman (1948) and Penman (1963) suggested using values equivalent to 1 and 0.537 for a, and b,, respectively, for a short grass cover when u is measured in metres per second (Brutsaert, 1982). The equation used to estimate net radiation (R,) from solar radiation is R, = (1 - r) R, - aqj(0.34

- 0.44( e,)“‘)(O.lO

+ 0.90n/N)

(10)

where I is the surface albedo, taken as 0.25 for short green cover, R, is the incident short-wave solar radiation (mm day-’ water equivalent), cr is the Stefar-Boltzman

Table 2 Monthly values of soil moisture deficit CD,,, Station

for the period 1960- 1987

D,,, .-.. (mm) Jan.

Tymbaki Ierapetra Heraklion Chania Kythera Rhodes Milos Methoni Kalamata Naxos Tripoli Elliniko Andravida Pyrgos Argostoli Chios Aliartos Agrinio Lamia Mytilini Voles Kerkyra Trikala Larissa Ioannina Trikala Hem. Kozani Mikra Alexan/lis Serres Komotini

) for various stations calculated

0 0 0 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Feb. 0 0 0 0 0 0

11 0 0 11 0 8 0 0 0 0 0 0 0 0 8 0 0 0 0 0 2 0 0 0 0

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sept.

Oct.

Nov.

Dec.

12 23 34 0 21 1 39 3 0 48 0 32 0 0 0 3 4 0 8 1 27 0 0 12 0 0 12 14 7 12 2

46 66 63 12 62 27 89 22 3 99 3 85 5 2 9 23 30 5 38 14 73 2 2 41 0 10 11 42 31 42 9

106 132 136 65 125 86 136 70 44 141 33 145 45 37 64 79 85 34 89 61 102 31 33 73 3 44 55 76 64 79 30

177 209 190 133 180 173 179 124 118 179 95 190 110 99 115 145 142 93 145 132 151 94 98 133 26 84 102 128 112 117 76

238 293 225 174 204 230 207 166 168 204 147 224 155 159 161 207 185 139 171 193 186 151 159 166 71 138 129 164 157 154 127

223 269 209 170 195 229 193 143 160 194 153 211 151 150 151 202 165 146 154 189 268 139 149 153 84 120 120 145 165 125 141

146 180 131 110 140 165 143 104 102 144 93 146 86 106 87 125 110 73 103 132 161 29 89 77 31 91 69 84 96 86 76

52 81 28 2 61 46 64 3 0 71 0 51 0 0 0 67 4 0 5 55 22 0 0 15 0 0 15 22 32 17 10

00 20 22 0 0 0 15 00 00 30 0 11 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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Water Management 32 (1996) 8.5-104

91

constant (mm day-’ water equivalent KW4), T, is the mean air temperature (K) and n/N is the ratio of actual to possible hours of sunshine. The incident short-wave solar radiation R, is estimated as

R, = R,( a + bn/N)

(11)

where R, is extraterrestrial radiation (received at the top of the atmosphere; mm day-’ water equivalent), and a and b are coefficients appropriate for various locations in Greece, given by Flocas (1980).

2Y3, lOANNlN.4

KALAMATA

250

0 JFMA

UJJ**ONC

MONTHS

250

KOMOTINI

MMlLlNl

250

* -P

s

0

N

0

DE+~W PET,,

-

DP.n

-*-

AETp,

Fig. 2. Monthly values of precipitation, potential evapotranspiration according to the Penman method and corresponding monthly values of actual evapotranspiration, and soil moisture deficit for selected stations.

P. Kerkides et al./Agriculturul

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Water Management 32 (1996) 85-104

2.3.1. Thornthwaite method method (Rosenberg et al., 1983) for estimation The Thornthwaite potential evapotranspiration (PET,,; mm) may be written as PET,,

= 16( Z&2)(

N/30)(

of monthly

10T,/I)a’

(12)

where I, is actual day length (h), N is the number of days in a month, T, is the mean monthly air temperature (“C> and (Y1 is an empirical coefficient defined as (Y, = 6.75 x 1O-713 - 7.71 X 1O-5Z2 + 1.79 x lo-*/

250

SERRES

+ 0.49

(13)

L4RlSSA

25.0

1

UJJ*SONO MONTHS

JFUA

-P

CK+CKWPETp.,

-

OPe.7

H*-

AUp,

Fig. 3. Monthly values of precipitation, potential evapotranspiration according to the Penman method and corresponding monthly values of actual evapotranspiration, and soil moisture deficit for selected stations.

P. Kerkides et al./Agricultural

Water Management 32 (1996) X5-104

where I is a heat index derived from the sum of 12 monthly from i = ( Ta/5)‘.5’4

index values,

93

i, obtained

(14)

3. Data analysis In this study, mean monthly values of dry and wet bulb temperatures, wind speed and actual hours of sunshine, measured at 31 meteorological stations throughout Greece are employed in Eqs. (7) and (12) to estimate mean monthly values of potential evapotranspiration PET,, and PET,, by using either the Penman or the Thomthwaite methods, respectively. Data are available for 27 part years (1960-1987) and the stations, located between latitudes 34” and 41”N are chosen from the stations of the network of the National

Fig. 4. Soil moisture deficit isolines (mm year-

’) for four stations representing different areas of Greece.

Rhodes Milos Methoni Kalamata Naxos Tripolis Elliniko Andravida Pyrgos Argostoli Chios

Tymbaki Ierapetra Her&lion Chania Kythera

Station

518 511 523 618 544 712 411 765 816 388 835 384 851 11020 858 579

(mm)

P

1005 1050 967 971 903 1003 893 906 931 903 779 993 906 916 916 941

(mm)

PET,,

1518 1784 1581 1275 1532 1621 1487 1400 1281 1509 1192 1489 1158 1240 1312 1430

(mm)

PET,,,

572 575 484 456 425 506 482 324 376 515 283 662 383 395 355 496

Dam

1000 1273 1038 666 988 957 1076 635 595 1121 524 1105 527 553 587 851

Dads

Estimated WCRZ for each station (mm)

1000 1273 1038 657 988 909 1076 635 557 1121 454 1105 472 497 516 851

300

1000 1273 1038 681 988 1000 1076 677 642 1121 533 1105 552 578 599 853

200

Fixed WCRZ for all stations (D rEN ; mm)

1273 1038 729 988 1050 1076 724 690 1121 579 1105 598 625 646 902

1000

150

1040 1303 1038 779 1000 1100 1076 774 739 1121 628 1105 647 674 695 952

100

Table 3 Annual precipitation (P 1, potential evapotranspiration obtained by the Thomthwaite (PET,) or Penman (PET PEN) method and the corresponding soil moisture deficits (D, and D,,, ) calculated from estimated water capacities of the root zone (WCRZ) for each station. Annual amounts of D,,, are also shown as calculated from WCRZ fixed at 300, 200, 150 or 100 mm for all stations

Aliartos Agrinio Lamia Mytilini Voles Kerkyra Trikala Larissa Ioannina Trikala Hem Kozani Mikra Alex/polis Serres Komotini

593 989 582 697 490 1138 744 426 1129 567 531 473 567 472 676

916 930 909 927 924 911 901 873 790 811 751 867 833 846 836

1318 1214 1295 1474 1488 1168 1216 1096 959 1054 1046 1148 1231 1104 1147

429 356 354 410 434 321 362 447 164 309 220 394 292 374 266

725 490 713 777 998 446 530 670 215 487 515 675 664 632 471

725 426 713 777 998 376 472 670 204 487 515 675 664 632 471

725 503 713 844 998 44.6 548 670 256 487 515 675 664 632 488

725 548 713 893 998 493 594 670 291 524 515 675 664 632 533

768 597 728 943 998 541 643 670 334 572 515 675 701 632 581

96

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Wuter Management 32 (1996) 85-104

Meteorological Service to represent a wide variety of meteorological conditions in the country. Characteristics of the stations are presented in Table 1. The program WTRBLN (Danker, 1987) is employed to calculate water balance according to the method of Thornthwaite and Mather (1957). This program may take into account direct runoff and provides a successive approximation method when the area climate is so dry that the soil never reaches field capacity. 3.1. Water balance 1960-1987 In the first part of the analysis the water capacities of the root zone (WCRZ) for soil-crop combinations used in WTRBLN are estimated (Eq. (2)) for each station according to Thomthwaite and Matber’s table (Dtmne and Leopold, 1978) and are

Table 4 estimated by the Thomtbwaite method (PET, > and soil Annual precipitation (P), potential evapotranspiration moisture deficit CD,,,) for the periods 1931- 1941 and 194% 1968 (Karras, 1973) and 1969- 1987 (present study) Station

Ierapetra Heraklion Chania Kythera Rhodes Miles Methoni Kalamata Naxos Tripolis Elliniko Pyrgos Argostoli Chios Aliartos Agrinio Lamia Mytilini Voles Kerkyra Trikala Larissa loannina Kozani Mikra Alex/polis Serves Komotini

1931-1941

and 1948-1968

1969- 1987

PET,

D300

kd

(mm)

(mm)

Cpmd

(mm)

571 499 656 587 748 489 776 812 397 901 416 831 1005 70.5 688 1027 694 729 476 1309 787 474 1239 594 443 578 590 665

908 931 958 942 969 918 917 908 914 740 967 902 932 958 897 925 930 940 918 890 896 838 772 743 854 788 834 777

421 432 357 390 410 430 311 295 517 175 551 284 292 426 318 272 290 407 442 218 296 363 120 148 413 244 244 173

489 476 666 522 660 402 730 811 408 805 382 1014 851 432 578 942 576 680 517 1099 737 417 1067 502 457 538 454 669

1030 944 963 888 987 876 890 893 896 767 969 899 911 884 909 912 903 914 917 893 895 861 776 737 851 820 840 821

PETTH

541 468 393 372 463 474 322 330 488 231 587 336 300 452 376 303 327 387 400 260 313 444 157 235 394 282 386 222

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32 (1996) 85-104

97

L?.xIS.sA

g

-20

?-30

+ +

NAXOS

r

50-

:

40'

8

30y

HETHONI

TXIKALA

Fig. 5. Annual variation of differences in potential evapotranspiration, precipitation and soil moisture deficit calculated in the present study (1969- 1987), and by Karras (1973) (1948- 1968): dotted line, dPET; thin line, d P; thick line, dDEF.

98

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Water Management 32 (1996) S-104

presented in Table 1. These average values were obtained for each station from vegetation characteristics and spatial distribution provided by the National Statistical Service of Greece (1986) and soil textures by the Soil Science Division of the National Institute of Agricultural Research (S. Aggelides, personal communication, 1986) which are also shown in Table 1. The estimates of potential evapotranspiration by the Penman method and the above mentioned estimates of WCRZ are used in WTRBLN to obtain monthly values, presented in Table 2, of soil moisture deficits CD,,, defined as PET - AET) for the period 1960- 1987. The average monthly totals of precipitation, potential evapotranspiration according to the Penman method and the corresponding monthly values of actual evapotranspiration and moisture deficit, are shown in Figs. 2 and 3 for selected stations (Ioannina, Kalamata, Komotini and Mytilini in Fig. 2; Serres, Larissa, Her&lion and Naxos in Fig. 3). The annual amounts of D,,, for various stations in Greece are illustrated as soil moisture deficit isolines in Fig. 4. In the second part of the analysis the calculations are also carried out using in WTRBLN the above mentioned estimates of WCRZ and Thomthwaite predictions for evapotranspiration, in order to evaluate the effect of a different evapotranspiration method on the final water balance estimates. The annual amounts of potential evapotranspiration obtained using the Thomthwaite (PET,,) and Penman (PET,,,) methods and the corresponding annual soil moisture deficits (D,, and D,,,) for the period 1960- 1987 are presented in Table 3. Finally, values of WCRZ (100, 150, 200 or 300 mm) fixed for all stations and Penman evapotranspiration estimates are used in WTRBLN to evaluate the effects of various soil-crop combinations in the annual amounts of soil moisture deficit. Such values of D,,, for each station are shown in Table 3. 3.2. Comparison

of water balance in various periods

Karras (1973) in a climate classification of Greece, estimated water balance over Greece, according to the method of Thomthwaite and Mather (1957) for the periods 193 1 - 1941 and 1948- 1968. He employed in his calculations a fixed value of 300 mm as the water capacity of the root zone for all stations and Thomthwaite evapotranspiration estimates. In order to present some comparisons of water balance values estimated in different periods, an identical analysis was carried out on the data covering the period (1969-1987), using in WTRBLN 300 mm as a fixed value for the water capacity of the root zone for all stations and Thomthwaite estimates as the potential evapotranspiration. The annual amounts of soil moisture deficits (D,,,) obtained and the corresponding annual amounts of precipitation P and potential evapotranspiration PET,, are shown in Table 4. The results obtained by Karras (1973) for the periods 1931-1941 and 1948-1968 are also presented in Table 4. The annual variation of differences in potential evapotranspiration (PET,, 1, precipitation (P) and soil moisture deficit (D,,,) calculated in the present study and by Karras (1973) are given in Fig. 5 for selected stations (Larissa, Heraklion, Naxos, Methoni, Komotini, Elliniko).

P. Kerkides et al./Agricultural

4. Results

Water Management 32 (1996) 85-104

99

and discussion

4.1. Present day water balance Attention has long been given to methods and techniques to obtain correct estimates of water balance components. In addition to very sophisticated methods, the method described by Thomthwaite and Mather (1957) is still in use because of its simplicity and its low requirements for input data. It is always advisable to study the accuracy of a method and, more importantly, to investigate whether it has produced useful results. Thus, it must be pointed out that in most published studies (especially climatological ones), estimates of water balance components are obtained from the original method, with potential evapotranspiration (PET) calculated from the equation of Thomthwaite (1948) and with the water capacity of the root zone (WCRZ) fixed at either 100 or 300 mm. It is obvious that more accurate water balance components can be achieved by considering more realistic estimates of PET based on Penman’s equation (which takes into account a more complete range of meteorological observations) and more precise water capacities of the root zone, especially calculated for each place according to local soil characteristics and crop area1 distribution. These two modifications of the original method (the purpose of which was to obtain more realistic estimates of water balance components for various areas in Greece), allowed their effects on the final estimates of soil water deficits (D) to be studied under the wide range of meteorological conditions, vegetation characteristics and soil textures existing throughout the country. Hence, taking into account the same sample of data (1960- 1987) and using the estimated WCRZ for each station, the effects of different methods of calculation of PET on soil moisture deficit are shown in Table 3. The differences between D,, and D,, are very high (23-56%) while the observed differences in PET are of the order of 8-48%. The use of WCRZ, fixed for all stations and varying from 100 to 300 mm resulted in unimportant to large differences in the estimated annual amounts of D,, (Table 3) depending on the station. WCRZ combines soil water holding capacity characteristics with the effective rooting depth of the vegetation of the area. It is an important parameter because it may control actual evapotranspiration, but above all, it controls groundwater runoff. Thus, according to the model, as soon as soil water stored in the soil profile, due to precipitation, exceeds WCRZ, deep percolation (e.g. groundwater runoff> takes place. The way this excessive water leaves the soil profile to meet the natural drainage channels of the catchment is usually taken as a percentage of the surplus water. A value of 50% is recommended (Thomthwaite and Mather, 1957; Dunne and Leopold, 1978) but it may be obtained more accurately from long-term observations of streamflow of the catchment. It is evident from the above that, for some stations, (dry stations), soil water stored in the soil profile may never reach field capacity and therefore groundwater runoff may not be observed, regardless of the value of WCRZ. In this case, annual estimates of soil moisture deficits will not be affected by varying WCRZ values as long as these values preclude the possibility of groundwater runoff. Thus, the final estimates of D,, in some stations (He&lion, Milos, Naxos, Elliniko, Volos, Larissa, Kozani, Mikra, Serres) are not affected by varying WCRZ from 100 to

100

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300 mm. These stations have received quite small annual amounts of rainfall (in almost all of them, P < 500 mm) inducing very low amounts of groundwater runoff, even during the wet period. It must be mentioned though, that some differences are observed even in these stations when the interannual patterns are examined. When the range of variation of WCRZ is reduced to 150-300 mm, some more stations exhibit no change in the annual amount of D,,, (Kythera, Ierapetra, Tymbaki, Lamia, Aliartos, Alexandroupolis). These stations already present very low variations in D,,, (l-6%) when WCRZ is considered being changed from 100 to 300 mm. In other cases, where soil water reaches field capacity, various values of WCRZ will result in differences in soil moisture deficits both interannually and annually. Differences of 33-64% are obtained in some rather humid stations (Kalamata, Argostoli, Trikala, Pyrgos, Andravida, Tripolis, Agrinio, Kerkyra, Ioannina). The rest of the stations (Chios, Trikala, Chania, Rhodes, Mytilini, Methoni, Komotini) show changes in DPEN from 12 to 23%. The variation of annual amounts of soil water deficits throughout Greece is illustrated in Fig. 4. The soil moisture deficit isolines provide a realistic picture of the mean annual D PEN distribution over Greece, in accordance with general geographical and soil-vegetation characteristics. It is apparent that the smallest soil moisture deficits are observed in the mountainous region of western Greece, where rainfall is very high and potential evapotranspiration very small owing to low temperatures (as a result of the altitude) and low wind speeds even at high altitudes (mountain ranges present an obstacle for prevailing high wind speeds). More than 22% of the land is covered by forests (especially in the continental areas), a percentage which is considered the highest in Greece. Pasture also covers an important percentage of the land in this area. Generally, a lower soil moisture deficit is observed, even in areas in western Greece which are of low altitude. These lower values are apparent throughout the western part of the country, from north to south (Crete). Rainfall is higher in these areas than in eastern Greece. The east coast shows increasing soil moisture deficit with decreasing latitude, owing to the higher temperatures and duration of sunshine and hence to the higher values of potential evapotranspiration. In contrast, the eastern islands have higher amounts of rainfall and hence smaller soil moisture deficits, owing to the quite humid air masses passing through the Aegean Sea. Forests and pastures are also an important percentage of the land in this area. The highest soil moisture deficits are observed on the eastern coasts of central and southern Greece (especially in southern Kyklades and eastern Crete) owing to low rainfall and mainly to high values of potential evapotranspiration. These highest annual amounts of potential evapotranspiration are due to the high wind speeds prevailing during summer when the etesians (local winds) are apparent. Only a very small percentage of land in this area is covered by forests. As shown by the mapping of soil moisture deficits over Greece (presented as monthly means in Table 2) maximum values of soil moisture deficit (D,,,) appear in July, corresponding to maximum values of potential evapotranspiration (PETPEN). However, some of the stations in western Greece (Ioannina, Agrinio, Tripolis) and eastern continental Greece (Alexandroupolis, Komotini, Voles) present their maximum values in August. This can be explained by the minimum precipitation being in August (instead of July, as in most of the Greek stations) and by values of potential evapotranspiration which remain very high in August. Most of the stations exhibit moisture deficit from

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March. Very few stations have soil moisture deficit from February and it is very rare to have soil moisture deficit from January (Ellinikol. This deficit remains up to October, or November in some cases. Soil moisture deficit does not occur at all stations examined during December. The passing of the depressions during the cold period (usually from November to February) causes a sufficient amount of precipitation in most of the stations. Rainfall in most areas in Greece is mainly due to the passing of these depressions. Therefore, western areas (of high or low altitude), being the first to receive the air masses after their long passing through the Mediterranean and hence their enrichment with water vapours, show a late deficit in April or even later (i.e. Ioannina). A different presentation of water balance components is shown for eight stations, selected to provide two broad clusters with respect to precipitation. Ioannina, Kalamata, Komotini and Mytilini represent the wetter stations (Fig. 2) and Serres, Larissa, He&lion and Naxos the drier stations (Fig. 3). The first four stations are characterised by high values of WCRZ (236-284 mm>, and those for the latter four stations are among the lowest in Greece (172-203 mm). Ioamrina is the main representative of the very wet high altitude stations in western Greece, where the highest amounts of rainfall, the smallest values of potential and actual evapotranspiration and hence the smallest soil moisture deficits are apparent. Kalamata, representing western stations with low altitude and quite high rainfall, has a higher potential and lower actual evapotranspiration and consequently less available water than Ioannina. During the warm period especially, the soil moisture deficit becomes high. In Komotini (a representative of low altitude, north eastern stations), the high amounts of rainfall even during the warm period result in actual evapotranspiration being equal to rainfall and a small water deficit, although the potential evapotranspiration is quite high, especially during the warm period. Mytilini, representing the islands in eastern Greece, with rainfall during the cold period (due to the passing of depressions over the Aegean Sea), shows important water deficits during the warm period, since potential evapotranspiration is high at that time and the amount of available water is very small. Among the dry stations, Serres represents low altitude areas in central and northern Greece. The annual pattern of rainfall is quite different in these areas. A low amount of rainfall is apparent throughout the year. It still remains during the warm season, due to the existence of thermal storms. However, potential evapotranspiration is high, actual evapotranspiration is equal to rainfall and consequently, the soil moisture deficits are quite high. Similar characteristics are apparent in Larissa, the representative of the stations in central Greece which are not affected by the sea due to the existence of physical obstacles or their distance from it. Heraklion and Naxos, representing the islands in the south and central Aegean Sea, respectively, have very high soil moisture deficits (especially during the warm period) mainly attributed to the existence of zero rainfall rates and very high potential evapotranspiration as a result of the prevailing local winds (‘etesians’) at that time. 4.2. Comparison

of water balance in various periods

The annual potential evapotranspiration method by Karras (1973) for the periods

calculated 193 l-1941

according to the Thomthwaite and 1948-1968, are similar to

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those obtained in the present study for the period 1969-1987 (Table 3). The differences are less than 6% and are probably due to slight differences in temperature and, to a lesser extent, the computational facilities used. Thus, PET,, does not seem to show any important variation from one period to the other. Twenty-two stations show lower annual rainfall during recent years, but there are six stations at which precipitation is higher (during the years 1969-1987). The annual soil moisture deficits (Ds,,,,) are generally increased during recent years, corresponding to the variations in potential evapotranspiration and precipitation. Sometimes this increase appears to be important (increases of 22% and 37% were observed in Komotini and Serres, respectively). However, there are stations in various areas of the country where annual D,,,, has decreased during recent years, corresponding to a decrease in annual PET,, . It must be mentioned here that a small variation in PET,, can cause a noticeable variation in D,,,. Generally, the annual variations of PET,, , P and OjoO obtained for various stations in Greece do not indicate any great differences compared with those estimated by Karras (1973). The annual variations of these differences (dPET, dP and dDEF, respectively) do not show systematic behaviour which holds throughout Greece. Four stations (Larissa, Naxos, Methoni and Trikala) have been selected to illustrate various representative differences in annual variations of OsO,,, PET,, and P estimated between the two periods mentioned (Fig. 5). Thus, Larissa and Methoni represent stations with positive values of dDEF during the warm period (large differences in Larissa, small ones in Methoni). Naxos represents stations with negative values of dDEF during most of the year, while Trikala represents stations with positive and negative values of dDEF throughout the year. The PET,, value obtained for 1969-1987 seems to be slightly greater and smaller than that estimated for the previous period during spring/summer and autumn/winter, respectively. Recent values of P are often greater than previous values for various months of the year. These differences are mainly apparent during the wet season and hence do not affect the final estimates of D,,, during this period.

5. Conclusions Accurate calculations of water balance components can be achieved by replacing the Thomthwaite equation in Thomthwaite and Mather’s original method by potential evapotranspiration values estimated using the Penman equation and by the use of more appropriate water capacities of the root zone, calculated for each area according to local soil characteristics and crop area1 distribution. This study evaluated the effects of these two modifications to Thomthwaite and Mather’s method on final estimates of soil water deficits under the wide variety of meteorological conditions, vegetation characteristics and soil textures existing throughout Greece. This method of calculation of evapotranspiration may lead to large differences in annual soil moisture deficit, of the order of 23-56%. Values of WCRZ varying from 100 to 300 mm may result in differences in annual soil moisture deficit of O-64%. Thus, depicting soil moisture deficits countrywide, both annually and interannually, may prove to be a useful tool for more rational regional-scale water management planning if the model is provided with the most accurate input parameters possible.

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This study has shown that western Greece suffers the least soil moisture deficit throughout the year, while central and southern regions, together with the Aegean Islands, suffer the most. Comparison of water balance results, obtained by applying the model to identical input parameters (e.g. same values of WCRZ, same methodology for estimating PET, AET, storm or groundwater runoff etc.> for two different time periods showed slight differences towards a drier period which may not be indicative of an important diachronic climate change.

Acknowledgements

The authors wish to express their thanks to the National Meteorological Service and the Soil Science Division of the National Institute of Agricultural Research.

References Belmans, C., 1985. SWATRER Ref. Manual, Laboratory of Soil and Water Engineering, Katholike Universiteit, Leuven, Belgium, 114 pp. Belmans, C., Wesseling, J.G. and Feddes, R.A., 1983. Simulation model of a water balance of a cropped soil: SWATRE. J. Hydrol., 63: 271. Brutsaert, W.H., 1982. Evaporation into the Atmosphere. Reidel, Boston, MA. Donker, N.H.W., 1987. WTRBLN: A computer programme to calculate water balance, Comput. Geosci., 13 (2): 95- 122. Dragoni, W., 1993. Response of some hydrological systems in central Italy to climatic variations. Presented at the NATO Advanced Research Workshop on Diachronic Climatic Impacts on Water Resources, Iraklio, Greece, 17-23 October 1993. Dunne, T. and Leopold, B.L., 1978. Water in Environmental Planning, W.H. Freeman, San Francisco, CA. Flocas, A.A., 1980. Estimation and prediction of global solar radiation over Greece. Solar Energy, 24: 63-70. Issar, A., 1993. Climate variations during the historical period in the eastern Mediterranean region. Presented at the NATO Advanced Research Workshop on Diachronic Climatic Impacts on Water Resources, Iraklio, Greece, 17-23 October 1993. Karras, G.S., 1973. Climate classification of Greece, according to Thomthwaite. Ph.D. Thesis, Athens University. (In Greek.) Kypris, D., 1993. Cyclic climatic changes in Cyprus as evidenced from historic documents and one century’s rainfall records. NATO Advanced Research Workshop on Diachronic Climatic Impacts on Water Resources, Iraklio, Greece, 17-23 October 1993. Kypris, D., 1995. Diachronic changes of rainfall and the water resouces in Cyprus. In: Tsiourtis (Editor), Water Resources Management under Drought or Water Shortage Conditions, Proc. EWRA 95 Symposium, Nicosia, Cyprus, 1418 March 1995. Balkema, pp. 1 l-18. Milliman, J.D., Jeftic, L. and Sestini, G., 1992. The Mediterranean Sea and climate change-an overview. In: L. Jeftic, J.D. Milliman and G. Sestini (Editors), Climate Change and the Mediterranean. Edward Arnold, London, pp. 1-14. Monteith, J.L., 1981. Evaporation and surface temperature. Q. J. R. Meteorol. Sot., 107: l-27. National Statistical Service of Greece, 1986. Distribution of the country’s area by basic categories of land use (Presencus data of the Agriculture-Livestock Census of the Year 1981). Agriculture-Forestry etc., Athens. Palutikof, J.P., Goodess, C.M. and Guo, X., 1994. Climate change, potential evapotranspiration and moisture availability in the Mediterranean basin. Int. J. Climatol., 14: 853-869.

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Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Sot. London Ser. A, 193: 120-145. Penman, H.L., 1963. Vegetation and hydrology, Tech. Commun. No. 53, Commonwealth Bureau of Soils, Harpenden, UK. Rosenberg, N.J., Blad, B.L. and Verma, S.B., 1983. MICROCLIMATE. The Biological Environment. Wiley, New York. Thomthwaite, C.W., 1948. An approach toward a rational classification of climate. Geogr. Rev., 38: 55-94. Thomthwaite, C.W. and Mather, J.R., 1955. The water balance. Climatology, 8: l- 104. Thomthwaite, C.W. and Mather, J.R., 1957. Instructions and tables for computing potential evapotranspiration and the water balance. Publ. No. 10, Laboratory of Climatology, Centerton, NJ.