N N 3 1 5 4 5 . 0 8 9 0 D T A 8 9° n o v e m b e r 1975
Instituut voor Cultuurtechniek en Waterhuishouding Wageningen
WORKABILITY AND DRAINAGE
i r . G. P . Wind
BIBrtLBö'ïHtÉks:N o t a ' s van het Instituut zijn in p r i n c i p e i n t e r n e c o m m u n i c a t i e m i d d e -l e n , dus geen officië-le p u b -l i k a t i e s .
Hun inhoud v a r i e e r t s t e r k en kan zowel b e t r e k k i n g hebben op een eenvoudige w e e r g a v e van c i j f e r r e e k s e n , a l s op een concluderende d i s c u s s i e van o n d e r z o e k s r e s u l t a t e n . In de m e e s t e gevallen zullen de c o n c l u s i e s e c h t e r van voorlopige a a r d zijn o m d a t het o n d e r z o e k nog niet is afgesloten.
Bepaalde n o t a ' s komen niet voor v e r s p r e i d i n g buiten het Instituut in a a n m e r k i n g
| 5 W / O / u ^ 3 : |||Ä
C O N T E N T S page INTRODUCTION 1 METHOD 2 RESULTS 2 APPLICATION 3 LITERATURE 5 FIGURES 1-7
INTRODUCTION
F e r t i l i t y of a s o i l , both p h y s i c a l and c h e m i c a l , i s c e r t a i n l y an i m p o r t a n t factor in d e t e r m i n i n g suitability for a g r i c u l t u r e . F o r m o d e r n a g r i c u l t u r e the workability i s a l r e a d y i m p o r t a n t and its s i g -nificance i s s t i l l growing. In the beginning of t h i s c e n t u r y land p r i c e s of the f e r t i l e , poorly workable clay s o i l s w e r e twice those of the i n -f e r t i l e , well workable sandy s o i l s . Today one can buy two ha clay soil for the selling p r i c e of one ha sandy soil in the N e t h e r l a n d s .
Workability h a s been i n v e s t i g a t e d l e s s than fertility. Workability is affected by a l a r g e n u m b e r of f a c t o r s , e. g. d r a i n a g e , s t r u c t u r e , soil profile, p r e c e d i n g c r o p . But the m o s t i m p o r t a n t factor g o v e r n -ing workability is w h e a t h e r . Its influence is so big that one cannot succesfully study the effect of other f a c t o r s if not the influence of w h e a t h e r is i n v e s t i g a t e d f i r s t . This h a s s e l d o m been done b e c a u s e of two r e a s o n s : f i r s t l y it was i m p o s s i b l e to c a l c u l a t e m o i s t u r e c o n -ditions of the topsoil from w h e a t h e r data; secondly it i s i m p r a c t i c a l r e s e a r c h , for the w h e a t h e r cannot be c o n t r o l l e d .
Now that dynamic m o d e l s a r e a v a i l a b l e of the u n s a t u r a t e d flow of m o i s t u r e , the f i r s t r e a s o n has fallen away. It is now p o s s i b l e to c a l c u l a t e m o i s t u r e condition, and thus w o r k a b i l i t y , o v e r a l a r g e n u m b e r of y e a r s . And so it h a s b e c o m e p o s s i b l e to investigate the effect of m a n y f a c t o r s on w o r k a b i l i t y , with n a t u r a l or s t a n d a r d i z e d w h e a t h e r conditions.
This p a p e r d i s c u s s e s the effect of d r a i n d e p t h and d r a i n a g e i n -t e n s i -t y on workabili-ty of a l o a m s o i l in s p r i n g . F o r -the inves-tiga-tion n a t u r a l w h e a t h e r conditions w e r e u s e d , and it s e e m s now, that such r e s e a r c h is c e r t a i n l y not t o t a l l y i m p r a c t i c a l .
METHOD
M o i s t u r e tension in the top 5 c m was c a l c u l a t e d during 23 y e a r s in the m o n t h s J a n u a r y to the end of A p r i l , 5 t i m e s p e r day. T h e r e -fore an analog model of WIND (1972) was u s e d and two n u m e r i c a l m o d e l s of VAN KEULEN and VAN BEEK (1971) and of WIND and VAN DOORNE (197 5).
T h e s e m o d e l s can c a l c u l a t e the m o i s t u r e condition at e v e r y depth and t i m e by combination of the s t o r a g e function ( m o i s t u r e c h a r a c t e -r i s t i c ) and the t -r a n s p o -r t function of the u n s a t u -r a t e d soil with the d r a i n a g e function. The l a t t e r is e x p r e s s e d in depth and i n t e n s i t y .
The initial condition at J a n u a r y f i r s t was found by a rough c a l -culation over the p r e c e d i n g eight m o n t h s . The daily r a i n f a l l - and e v a p o r a t i o n data of the m e t e o r o l o g i c a l station in De Bilt w e r e u s e d of e a c h of the y e a r s between 1951 and 1973.
A d e s c r i p t i o n of the application of the m o d e l s , the p r o b l e m s a r i s i n g and t h e i r solution will be published by WIND, e a r l y in 197 6 in N e t h e r l a n d s J o u r n a l of A g r i c u l t u r a l S c i e n c e .
The c a l c u l a t i o n s could be checked by o b s e r v a t i o n s in all 23 y e a r s on the e x p e r i m e n t a l f a r m W e s t m a a s , fig. 1. The soil u s e d is a l o a m s o i l , thought to be h o m o g e n e o u s , containing 25% clay.
The soil was c o n s i d e r e d a s workable for planting when the top 5 c m had a m o i s t u r e tension of -100 c m for g r a i n s and of -300 c m for potatoes and s u g a r b e e t s or when it was d r i e r than these v a l u e s .
The workable p e r i o d s w e r e c a l c u l a t e d in 23 y e a r s for five d r a i n -depths and t h r e e d r a i n a g e i n t e n s i t i e s .
RESULTS
In f i g u r e s 2 and 3 the effect of d r a i n a g e i n t e n s i t y on total amount of w o r k a b l e days ( c r i t e r i o n : u> < -300 cm) before May f i r s t a r e shown for d r a i n depths 80 and 150 c m below s u r f a c e . A d r a i n a g e i n t e n s i t y
-1 -1 of 0, 01 day m e a n s that d r a i n a g e d i s c h a r g e r a t e is 1 c m . day if
the g r o u n d w a t e r table midway between two d r a i n t i l e s is 100 c m above the d r a i n s . A l i n e a r r e l a t i o n between d i s c h a r g e r a t e and hy-d r a u l i c heahy-d was supposehy-d to e x i s t .
T h e r e s e e m s to be but a s m a l l influence of d r a i n a g e i n t e n s i t y on w o r k a b i l i t y in s p r i n g . F i g . 3 shows c o n s i d e r a b l e influence below an
_1
i n t e n s i t y of 0, 003 day . But that h a s no p r a c t i c a l m e a n i n g . Thus p o o r l y d r a i n e d s o i l s a r e not fit for a r a b l e a g r i c u l t u r e b e c a u s e of i n -undations in w i n t e r . The design i n t e n s i t i e s u s e d in the N e t h e r l a n d ' s d r a i n a g e c r i t e r i o n a r e 0, 023 and 0, 007 day for r e s p . 80 and 150 c m d r a i n depth. A c c o r d i n g to WESSELING (1969) thus d r a i n e d s o i l s have a g r o u n d w a t e r depth of 25 c m o c c u r r i n g once p e r y e a r .
So in the a p p r o p r i a t e r a n g e of i n t e n s i t i e s the choice of the d r a i n -age i n t e n s i t y does h a r d l y affect workability in s p r i n g .
The effect of d r a i n depth is shown in fig. 4 and 5. T h e s e give the amount of days in which the top 5 c m was d r i e r than a m o i s t u r e
tension of 300 c m . In m a n y y e a r s the effect a p p e a r s to be c o n s i d e r a b l e . A single y e a r (1970) showed no workable days before May. Other y e a r s had few workable days combined with s m a l l effect of d r a i n depth (1959, 1962, 1966). Many y e a r s w e r e so d r y ( e . g . 1968) that even v e r y s h a l -low d r a i n a g e gave a lot of workable d a y s . In such y e a r s i n c r e a s e of w o r k a b i l i t y h a s not m u c h s e n s e . In about half of the y e a r s t h e r e is
c o n s i d e r a b l e influence of d r a i n depth in a r e l e v a n t range of workability.
APPLICATION
A probability d i s t r i b u t i o n of workable days in dependence of d r a i n -depth i s shown in fig. 6. This is of c o u r s e only valid for this l o a m soil and the w h e a t h e r of the u s e d 23 y e a r s in the N e t h e r l a n d s and for the w o r k a b i l i t y - l i m i t of vp = - 300 c m .
The higher the w o r k a b i l i t y , the s m a l l e r the r e q u i r e d a m o u n t s of labour and equipment can b e . F a r m e r s will choose these f a c t o r s so that in only few y e a r s (e. g. 2 out of 10) the n o r m a l s p r i n g w o r k s c a n -not be a c c o m p l i s h e d in due t i m e . F o r low p r o b a b i l i t i e s the workability at 150 c m d r a i n depth is about twice that of 80 c m depth. I n c r e a s i n g d r a i n depth from 150 to 200 c m looks not v e r y helpful.
More p r o b a b i l i t y d i s t r i b u t i o n s have been made from the m o d e l -output for e v e r y 5-day p e r i o d in M a r c h and A p r i l . This will be u s e d in an o p t i m i z a t i o n m o d e l by the Institute of F a r m M e c h a n i c s and L a b o u r (IMAG). F r o m thus obtained knowledge it will be p o s s i b l e to
c a l c u l a t e drainage benefits for m i n i m i z i n g f a r m c o s t s .
More workable days m e a n s also e a r l i e r workability in spring and that m e a n s e a r l i e r sowing and planting. P r i m a r i l y the e a r l i e r w o r k a b i l i t y of deeply d r a i n e d soils is c a u s e d by t h e i r lower m o i s t u r e content than that of shallow d r a i n e d s o i l s . This c a u s e s a difference of s o m e days in a dry p e r i o d . This difference is amplified s t r o n g l y by the i r r e g u l a r rainfall p a t t e r n .
F i g . 7 gives the workable p e r i o d s ( y < - 300 cm) during M a r c h and A p r i l for 4 d r a i n depths in 23 y e a r s . The y e a r I960 h a s been o m i t t e d b e c a u s e the chosen initial condition at J a n u a r y f i r s t could influence the r e s u l t s c o n s i d e r a b l y in that y e a r .
In e v e r y y e a r the soil was the e a r l i e r workable the d e e p e r it was d r a i n e d . In some y e a r s the difference was only s m a l l (1951), in o t h e r y e a r s v e r y big (1955).
The r e l a t i o n between sowing time and yield d e p r e s s i o n was i n v e s t i g a t e d by WIND (I960). With the aid of this paper and the w o r k -able p e r i o d s given by fig. 7 VAN WIJK and FEDDES (1975) calculated the yield d e p r e s s i o n s , c a u s e d by lack of w o r k a b i l i t y . They a s s u m e d that for sowing s u m m e r g r a i n s 5 days a r e r e q u i r e d p e r f a r m and that s u g a r b e e t and potatoes can be sowed ( r e s p . planted) t o g e t h e r in 5 d a y s . F u r t h e r they a s s u m e d that the f a r m e r will use the f i r s t 5 available workable d a y s .
The yield d e p r e s s i o n s thus c a l c u l a t e d and a v e r a g e d o v e r 23 y e a r s a r e given in table 1.
Table 1. Yield d e p r e s s i o n c a u s e d by too late sowing, as a function of d r a i n depth. The data a r e a v e r a g e d over the p e r i o d 1951-1973. After VAN WIJK and FEDDES
Drain depth c m 40 80 100 150 S u m m e r g r a i n % 36 18 12 8 P o t a t o e s and s u g a r b e e t % 12 7 6 4
The effect of d r a i n depth i s l a r g e r for s u m m e r g r a i n s than for p o t a t o e s and s u g a r b e e t . This can be explained by the e a r l y p e r i o d in s p r i n g in which the f o r m e r c r o p is sown. During that t i m e , F e b r u a r y and M a r c h , e v a p o r a t i o n is low and the amount of m o i s t u r e in soil h a s big influence on the t i m e which is r e q u i r e d to obtain w o r k a b i l i t y . P o t a t o e s a r e planted, and s u g a r b e e t s a r e sown not before M a r c h 20, b e c a u s e of frost h a z a r d . In that p e r i o d e v a p o r a t i o n is higher and
soil m o i s t u r e h a s l e s s influence.
Table 1 gives the opportunity to c a l c u l a t e a benefit of d r a i n a g e . A s s u m i n g that in 4 y e a r s 1 potato, 1 s u g a r b e e t and 1 s u m m e r g r a i n
-_1 c r o p o c c u r , and that the value of the yield is 2000 Hfl. ha for
_1
s u m m e r g r a i n s and 4000 Hfl. ha for the o t h e r c r o p s , the d r a i n a g e -1 -1
benefit is 250 Hfl. ha . y e a r in the r a n g e between 40 and 80 c m depth. F o r the r a n g e 80-100 and 100-150 c m , the benefit is 50 and
-1 -1
60 Hfl. ha. . y e a r r e s p e c t i v e l y .
T h e s e data a r e not sufficient, they have to be c o m p l e t e d with the effect of d r a i n a g e on f a r m c o s t s , on workability in a u t u m n , on w a t e r damage to c r o p s and soil in w i n t e r . M o r e o v e r d r a i n depth i s a factor in drought damage which m a y o c c u r in d r y y e a r s .
LITERATURE
KEULEN, H. VAN and C. G. E . M . VAN BEEK, 1971. Water m o v e -m e n t in l a y e r e d s o i l s - A s i -m u l a t i o n -m o d e l . Neth. J . A g r . S c i . 19: 138-153
WESSELING, J . , 1969. B e r g i n g s f a c t o r en d r a i n a g e c r i t e r i u m . ICW Mededeling 118
WIND, G. P . , I 9 6 0 . O p b r e n g s t d e r v i n g door te laat z a a i e n . Landb. T i j d s c h r . 72: 111-118
, 1972. A h y d r a u l i c m o d e l for the s i m u l a t i o n of non-h e s t e r e t i c v e r t i c a l u n s a t u r a t e d flow of m o i s t u r e in s o i l s . J. Hydr. XV: 227-246
and W. VAN DOORNE, 1975. A n u m e r i c a l model for the simulation of u n s a t u r a t e d v e r t i c a l flow of m o i s t u r e in s o i l s . J. Hydr. 24: 1-20
WIND, G. P . , 1976. Application of analog and n u m e r i c a l m o d e l s for an investigation of the influence of d r a i n a g e on w o r k a b i l i t y in s p r i n g . To be published in Neth. J. A g r . Sei.
WIJK, A. L . M . VAN and R. A. FEDDES, 1975. Invloed van de w a t e r -huishouding op de o p b r e n g s t van landbouwgewassen.
number of workable days x 14 o analog calculation x observation by Hokke 1 - 1 5 April 6 - 3 0 April 19 5T
Fig. 1. Observed and calculated numbers of workable days in 23 years
number of workable days 25 r 2 0 -15 10 X 0005 6 6 0,010 0,015 drainage intensity A in days'*
Fig. 2. Effect of drainage intensity on the total amount of workable days ( Y< - 300 cm) in March and April for drain depth 80 cm below surface
number of workable days 30
drainage intensity A in days"1
F i g . 3. As fig. 2 for d r a i n depth 150 c m below s u r f a c e
workable days 2 5 t -. -. - 4 6 6 I Sö eb ioÔ" 1*8 Ï5Ö 2 0 0 drainage depth (cm) 70
F i g . 4. Effect of d r a i n depth on the total amount of workable days (<f < - 3 00 cm) in M a r c h and A p r i l for the l a s t 8 y e a r s of calculation
workable days March and A p r i l 20
. 5 8
8 0 100 150 200
drainage depth (em)
Fig. 5. As fig. 4, for the years with poor workability of shallow drained soil
workable days
3 0 p drain depth (cm ) 200
150
Fig. 6. Probability distribution of total workability ( y < - 300 cm) in March and April
year
workable if drain depth > 4 0 c m
>100 >150 >2O0 51 52 5 3 5 4 55 56 5 7 5» 5 9 61 62 6 3 6 4 6 5 6 6 6 7 6 8 6 9 70 71 72 7 3 1 H • «LZI i i i i i i i i i i m : i
