ROCZNIKI GLEBOZNAW CZE, T. X X X II, NR 3, W ARSZAW A 1981
PIO TR HEW ELKE
INVESTIGATIONS ON FORMATION OF MOISTURE POTENTIALS IN SOIL AT FIELD WATER CAPACITY
In stitu te o f Im p rovem en t, A g ricu ltu ra l U n iv e r sity at W arsaw
For a correct estim ation of w ater relations in the soil profile the method of field w ater capacity is generally applied. It enables to determ ine the soil retention value as a basis for an appropriate w ater m anagem ent on irrigated areas. The field w ater capacity is defined usually as the m axim um w ater reserve in the soil profile, stabilizing after saturation of active soil layer w ith w ater and filtration of its surplus into the profile depth unless therm ic and m oisture gradients occurred. An exact determ ination of the field w ater capacity is difficult, as the am ount of retentioned w ater opposed to the gravity force is variable and depends, among other things, on initial soil m oisture and ground w ater table.
The values of m oisture potentials cited in the literature, corresponding w ith the field w ater capacity are characterized by a ra th e r wide span. According to R i c h a r d s [6], w ater in the aeration zone is stored in pores, in which the superficial tension and capillary forces am ount to about 344 cm of w ater column, i.e. pF = 2.54, w hat is frequently assumed as a value of field w ater capacity in Poland. In the N etherlands and France [2, 5] as the field w ater capacity the m oisture corresponding w ith the m aternal sucking of soil of 100 cm 6'f w ater column, i.e. pF = 2.0, is generally assumed. In investigations of T r z e c k i [7, 8], G i e d r o j ć {1] and K r ó l [4] the potentials corresponding w ith the field w ater capacity of pH = 1.9-2.8 were obtained. These values are, however, not fully comparable, in view of application by particular researchers of different methods of determ ination of the above param eter.
From the physical viewpoint, as the field w ater capacity the state should be assumed, at which the w ater-soil system rem ains in equilib rium , hence the sum of potentials of F components in every point of the soil profile equals zero, according to equation :
where:
F с — total potential, F g — gravity potential,
Fw — m aternal (moisture) potential, Fs — osmotic potential,
Ft — tem perature potential, Fp — pressure potential.
If to omit the Fs, Ft and Fp potential values as relativ ery little and to assume as a reference level the free ground w ater table, so under conditions of equilibrium , in every soil profile point will be :
Fg = Fw (2)
It means th a t the m aternal potential of soil at an arb itrary point of the profile equals to the geometric rise of the given point above the free w ater table (Figs 1, 2).
Fig. 1. T h eoretical d ep en d en ce of the soil m oistu re p o ten tia l (F) on the ground w a ter tab le (z) at th e soil p ro file m oistu re eq u ilib riu m
state
F ig. 2. S ch em e of the e x p erim en t at e x a m in g th e soil flo w - o ff 1 — PVC pipe, 2 — p ro tectio n again st ev a p o ra tio n and d ea era tio n , 3 — te n sio - m eters, 4 — tra n sp o rta b le w e sse l fo r esta b lish m en t of the w ater ta b le , 5 — co n tro lled m ea su rem en t o f flo w -o ff, 6 —
p iezo m ete r w ith v a lv e
From the practical viewpoint the knowledge of the m oisture state, which will occur in the soil profile after 2-3 days of its demands would be necessary. The experim ents presented and discussed in the fu rth er p art of the paper enabled to determ ine m oisture potentials forming during drainage and prove th a t stabilization of the equilibrium in the profile requires much time [3, 9].
The investigations on form ation of m oisture potentials were carried out under laboratory conditions on soil columns artificially packed, taken in an undisturbed structure and under field conditions in natu ral soil
Moisture potentials in soils 13
profiles. The m easurem ent of potentials during soil drainage at different w ater table states was carried out by means of tile tensiom eters arranged at the distances of 5-20 cm (Fig. 1). To avoid occurrence of the pheno menon of histeresis, which could to a considerable extent make difficult interpretation of the results, the soil drainage was started each time from the full saturation state.
The arrangem ent of potentials after flow-off lasting three days has been assumed as the state established from the practical viewpoint. While disposing of curves of w ater desorption for particular soil profiles, soil m oisture valeus corresponding w ith them were calculated and dif ferences of w ater reserves at the theoretical state (Fg = F w) and the state recorded after three days were determ ined. Investigation results.
Detailed results of the investigations are presented as an example for one among the soil profiles w ith known physical properties (Table 1). The w ater desorption curve of the profile in question is presented
T a b l e 1
b a sic ph ysical p ro p erties o f the sandy s o i l in v e stig a te d
Depth Bulk d en sity S p e c ificITT* Q T7 4 fir P orosity
Mcchanical composition p a r tic le s 0, % cm e/ca ? g/cm^ v o l. % 0 . 5 - 0 . 2 5Ш! 0 . 2 5 - 0 . 1mm 40 1.65 2.6 1 37 9 91 80 1 .6 3 2.61 33 9 91 120 ‘ 1.65 2.6 1 37 9 91
in Fig. 3. It follows from the potentials m easured during drainage (Fig. 4a) at the w ater table of 0.9 m below surface th a t the state observed after three days considerably deviated from theoretical one. The sucking power in the upper soil layer corresponded w ith pF==1.74. A fter eight days the sucking power increased up to pF =1.95, which should be stabilized at the equilibrium state. The flow-off differences at the state
Fig. 4. a — d istrib u tio n of p o ten tia ls at flo w - o ff
1 — state after t - 0 .7 day, 2 — sta te a fter t =1.7 day, 3 — sta te a fter t = 3 d ays, 4 — sta te after t = 8 d ays, 5 — th e o r e tic a l sta te
b — soil m oistu re d istrib u tio n at flo w - o ff 1 — state after t ~ 3 days. 2 — sta te after t = 8 d ays, 3 — th e o retic a l state
measured after three days and at the theoretical equilibrium state established on the basis of the soil m oisture distribution (Fig. 4a) am ounted to 17 mm.
Results obtained in rem aining soil profiles (Table 2) confirm the
T a b l e 2 S p e c ific a tio n o f valu es o f s o i l moisture p o te n tia ls In columns
and p o in ts in v e stig a te d a fte r three-day flo w -o ff
S o i l P o te n tia l
HgO cm pP hPa
A r t i f i c i a l l y packed p r o f ile s
Medium sand 33 1.52 32.4
Pine sand 9056, s o i l 10% 42 1,62 41.2
Fine sand 91%» medium sand 9% 55 1.74 53.9
Pine sand 85%, s o i l 10%, k a o lin ite 5% 26 1.42 25.5 P r o file s with n atu ral structure
Loose oand 56 1.75 54.9
Weakly loamy sand 52 1.72 51.0
Light loamy sand 60 1.78 58.8
Heavy loamy s i l t y sand on lig h t s i l t y loam 65 1.81 63.7
Light s i l t y loam 62 1.79 60 «8
Medium loom 72 1.86 70.6
P r o file s examined in natu ral co n d itio n s
Loose sand 56 1.75 54.9
Moisture potentials in soils 15
assum ption th a t the established soil m oisture state would occur practic ally at sucking power less than it would follow from the equation (2).
C O N C LU SIO N S
1. In investigations of the field w ater capacity in articicially packed and n atu ral soil profiles the m oisture potential of soil during flow-off stabilized to certain values and then slowly changed, not reaching theoretical values even after several teen days.
2. In artificially packed sandy soils the m oisture potential in the upper soil layer after three days of flow-off am ounted to p F = 1.42-1.74. In n atu ral soil profiles this potential am ounted for sands to pF==1.72- 1.81, for silty loam — to pH =1.79 and for medium loam — to pF =1.86.
3. The potential values obtained can be regarded as corresponding w ith practically established soil m oisture levels, but deviate from theoretical values.
4. Differences between w ater reserves in the soil profile after flow-off lasting three days (assumed as a standard at the field w ater capacity determ ination by the field method) and w ater reserves at theoretical distribution of m oisture potentials after the finished flow- off can be high, particularly in case of light soils. The above differences of w ater reserves in the soils under study am ounted to several — several ten m illim eters, depending on the w ater table and the pF curve course.
5. D eterm ination of field w ater capacity on the basis of w ater desorption curves at frequently assumed values of the m oisture potential pf p F = 2 .0 and the more of pF = 2.54, can lead to errors, since actually the values of the above potential are forming after practically significant drainage tim e at a lower level.
R EFERENCES
[1] G i e d r o j ć B. : W ater co n d itio n s in sandy soil under crops cu ltiv a ted w ith in a crop rotation . Zesz. nauk. W SR W rocław , R o ln ictw o , 26, 83, 1969.
[2] H o r s t v an der G. G., S t a к m a n W .P. : Soil m oisture retention curves. II. D irectio n s for th e u se of th e sa n d -b o x app aratu s, range pF 0-2.7. ICW W agen in gen , 1965.
[3] H e w e l k e P . : R a in fa ll d e fic ie n c y d eterm in a tio n at c a lcu la tio n of th e w a ter dem an d for irrigation of grasslan d s. D octor’s th esis, ty p escrip t, SG G W -A R , 1979.
[4] K r ó l H. : C om parison of sev era l m eth od s of th e fie ld w a te r cap acity d eterm in ation . R ocz. glebozn., A p p en d ix to th e v ol. 15, 1965.
{5] P e r i g a u d S. : C on trib u tion agron om iq u e a la m ise en v a le u r de la B renne. Q u elq u es a sp ects g én éra u x de p h y siq u e de sols. A n. A gr. 3, 1963.
[6] R i c h a r d s L. A. : P ressu re-m em b ran e apparatus construction and use. Agr. Eng. 28, 1974.
[7] T г z e с к i S. : In v e stig a tio n s on th e a b ility of w a te r reten tio n in soil. SGG W , 1967.
[8] T r z e с k i S., K r ó l H., S z u n i e w i c z J. : M ethods of d eterm in ation of d iffe r e n t w a te r ca p a titie s and d iffe r e n t p orosity of soils. PTG , Kom . F izy k i G leb (typescript), W arsaw 1971.
[9] Z a w a d a E., Ż a k o w i c z S., H e w e l k e P. : S oil su ction and w a te r flo w m ea su rem en ts in th e u n satu rated zone of sandy soil.
p . HEWELKE
B A D A N IA N A D K SZ T A Ł T O W A N IEM SIĘ PO T E N C JA Ł Ó W W ILG O TNO ŚCIO W YC H W G LEBIE PR ZY POLOW EJ
PO JEM N O ŚC I W OD N EJ
In sty tu t M elioracji i G ospodarki W odnej SG G W -A R w W arszaw ie
S t r e s z c z e n i e
P rzep row ad zon e bad an ia m ia ły na celu o k reślen ie w ie lk o ś c i p o ten cja łó w w il go tn o ścio w y ch , ja k ie k szta łtu ją się w g leb ie pod czas jej o d w ad n ian ia. S tw ierd zon o, że rozkład p o ten cja łó w w p ro filu g leb o w y m po cza sie od ciek an ia, m ającym zn a czen ie p ra k ty czn e (3 doby), odbiega od rozk ład u teo rety czn eg o od p ow iad ającego p olow ej p ojem n ości w od n ej. U zy sk a n e w a rto ści o m a w ia n y ch p o ten cja łó w w w ie r z c h
n iej w a r stw ie b ad an ych g leb w y n o siły od pF = l,42 do pF = l ,86. P on iew aż podczas d alszego o d w a d n ia n ia p o ten cja ł z m ie n ia ł się bardzo p o w o li, u zy sk a n e w artości m ożna uznać za o d p o w ia d a ją ce p ra k ty czn ie u sta lo n ej w ilg o tn o ści (połow a p o jem n ość w odna).
D r P to t r H e w e l k e
Iy is ty tu t M e lio r a c ji i G o s p o d a rk i W o d n e j S G G W - A R W a r s z a w a , ul. R a k o w i e c k a 26/32
