The movement of nitrate fertiliser from the soil surface to drainage waters by preferential flow in weakly structured soils, slapton, S. Devon

Agriculture, Ecosystems and Environment, 13 (1985) 241--259

241

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE MOVEMENT OF NITRATE FERTILISER FROM THE SOIL SURFACE TO DRAINAGE WATERS BY PREFERENTIAL FLOW IN W E A K L Y S T R U C T U R E D S O I L S , S L A P T O N , S. D E V O N

NIGEL COLES and STEPHEN TRUDGILL

Department of Geography, University of Sheffield, Sheffield $10 2TN ( Gt. Britain) (Accepted for publication 28 March 1985)

ABSTRACT Coles, N. and Trudgill, S., 1985. The movement o f nitrate fertiliser from the soil surface to drainage waters b y preferential flow in weakly structured soils, Slapton, S. Devon. Agric. Ecosystems Environ., 13: 241--259. Preferential flow of soil water down structural pathways can be responsible for the rapid movement o f a proportion o f surface-applied nitrate fertiliser to soil drainage waters. This phenomenon is most marked in cracking clay soils with strong structural development, but it has also been shown for a study site at Slapton in S. Devon, U.K., where weakly structured soils exist. The depth of recovery of surface-applied tracers, moving in preferential flow, was found to increase with increasing rainfall intensity. Soil outflow response was difficult to correlate with observed recovery depths due to the spatially variable nature of preferential flow and the consequent difficulties o f effecting an adequate soil sampling programme. Soil outflow response and surface-to-output linkages of preferential flow could, however, be characterised in terms o f combinations o f antecedent volumetric soil moisture values (e) and rainfall intensity. At 0 ~ 0.46 cm 3 cm -~ (field capacity), outflow response occurred over a wide range o f rainfall intensity; below 0 = 0.29, the mobile:retained soil water boundary, there was no response; between 0.29 and 0.46, response occurred mostly above an intensity threshold of 2.5 m m h -'. The management implications of nitrate movement under intense rainfall are briefly discussed.

INTRODUCTION P r e f e r e n t i a l f l o w in s o i l i n v o l v e s t h e r a p i d m o v e m e n t o f a p r o p o r t i o n o f the soil water down structural pathways during rainfall events. The signific a n c e o f s u c h f l o w is t h a t i t c a n b e r e s p o n s i b l e f o r t h e r a p i d t r a n s f e r o f surf a c e - a p p l i e d s o l u t e s , s u c h as n i t r a t e f e r t i l i s e r , t o d r a i n a g e w a t e r s , c a u s i n g b o t h l o s s e s t o t h e f e r t i l i s e d c r o p a n d a l s o u n d e s i r a b l e m o d i f i c a t i o n s o f agricultural drainage water quality. Most existing models of nitrate leaching do not allow for the occurrence of preferential flow, but predict the modal depth of nitrate leaching using uniform displacement models. The concern of t h e m o d e l s is o f t e n a l s o w i t h t h e d e p t h o f p e n e t r a t i o n o f s u r f a c e - a p p l i e d n i t r a t e w i t h r e s p e c t t o t h e r o o t i n g z o n e o f a c r o p , r a t h e r t h a n w i t h t h e leach.-

0167-8809/85/$03.30

© 1985 Elsevier Science Publishers B.V.

242 ing o f nitrate to the base of the soil profile and down-slope to inland waterways. In the present paper, an a t t e m p t has been made to characterise the existence of preferential flow in the study catchment area in S. Devon, and to study its significance for the rapid movement of nitrate fertiliser to streams. It is clear that in strongly structured soils, under conditions of intense rainfall, considerable quantities of input water move in a non-uniform manner (Thomas and Phillips, 1979), by-passing soil structures in increasing amounts as rainfall intensity increases (Bouma et al., 1981). Such by-passing flow in strongly structured clay soils has also been shown to be related to the surface to o u t p u t transfer of fertiliser nitrate under intense rainfall conditions (Smettem et al., 1983). In all these cases, uniform displacement models underpredicted the maximum penetration depth of surface-applied nitrate fertiliser travelling d o w n preferential pathways under intense rainfall. The interest, in the present paper, is to ascertain the extent to which preferential flow also occurs in weakly structured soils which are, perhaps, more widespread than strongly structured clay soils. In such strongly structured soils, existing work suggests that preferential flow is instigated because rainfall intensity values exceed the infiltration rates o f soil peds, the pedal infiltration excess travelling down inter-pedal pathways ( B o u m a et al., 1981, Trudgill et al., 1983a, b; Smettem and Trudgill, 1983; White et al., 1983, Kneale and White, 1984). Such an approach is more difficult to apply in weakly structured soils where the peds are difficult to define and to isolate for the purposes of studying their infiltration rates. It has thus proved difficult to provide predictive threshold data for the occurrence of preferential flow based on pedal infiltration values, although some attempt has been made to undertake this. The hypothesis tested in the present study is that the penetration d e p t h of surface-applied tracer will increase with rainfall intensity as the proportion of preferential flow increases -- as has already been shown for strongly structured soils. A second hypothesis under test is that soil outflow response, and the surface-to-output linkages b y preferential flow, will also increase with rainfall intensity. FIELD AREA AND METHODS The field area is at Slapton Ley Field Centre in South Devon, U.K., an area already noted for its high levels of nitrate losses in run-off water (Troake et al., 1976). The area has an average annual rainfall of 1076 mm, and the hydrological processes in the Slapton catchment have been described b y Burt et al. (1983). Steep slopes and permeable soils combine to provide large volumes of soil through-flow to stream channels, often with high nitrate concentrations (with peak levels recorded at around 15 mg 1-1). The soils are well~lrained acid brown earths, developed on Devonian slates, and t h e y have a silt loam or clay loam texture (Trudgill, 1983). The soils are loose and friable with weakly developed crumb or sub-angular b l o c k y structures, diffi-

243 cult to distinguish in field sections; well-marked biopores are, however, often present. Surface compaction frequently leads to higher bulk densities in the upper profile than in the lower, especially in permanent pasture. The study site was a sloping permanent grassland plot, 10 m wide and 100 m from slope base to slope crest. Within this major plot, study plots 60 cm X 60 cm were used to study the soil penetration depth o f surface applied tracers; 12 plots were used at the same topographic level and above a soil out-flow monitoring point. Antecedent soil moisture values were determined as the mean from replicate soil cores taken from 5--30 cm depth, with 10 replicates extracted prior to each rainfall event monitored. The cores were then dried at 105°C until constant weight was achieved, and the moisture c o n t e n t was expressed as the volumetric water content (0), cm s cm -s of the total soil volume. Soil moisture field capacity was determined in the laboratory for columns o f 19 cm diameter and 30 cm length after 48 h drainage from saturated conditions. An in situ field capacity value was also recorded for field soil subsequent to drainage events. Nitrate analyses were performed using a " C h e m l a b " autoanalyser, with determination o f oxidised nitrogen using hydrazine--copper reduction and an azo dye; tests showed nitrite levels to be negligible and the data are reported as mg 1-1 NOs-N. Chloride analyses were also performed using a mercuric thiocyanate colourimetric method on an autoanalyser, lSN analyses were performed on a mass spectrometer at A.R.C. Letcombe Laboratory. Soil extracts were obtained from 5-g soil samples shaken for 10 min with 15 ml deionised Water and centrifuged. Methylene Blue dye staining was used to demonstrate the existence of preferential flow in soils (Anderson and Bouma, 1977). Methylene Blue is a rapidly absorbed d y e which can be used for staining routeways of rapid water transmission in soils, at least near the source of application, since rapid absorption leads to attenuation of the tracer with soil depth. Two operations were involved. (1) For laboratory staining o f small soil samples (5--10 cm 3, or less, soil peds), the soil was embedded in paraffin wax with entry of stained water allowed from above b y a 1-cm diameter t u b e and free drainage allowed below. Staining was observed on sections o f the soil block. (2) In saturated field conditions, sections of 10- or 19-cm diameter plastic piping were lined with petroleum jelly to minimise edge flow and inserted vertically into the soil. Methylene Blue was placed in a ponded head o f water above the soil surface and allowed to infiltrate into the soft. Core sections were taken b y successively extruding the soil from the core and slicing the soil parallel to the soil surface at 1-cm intervals. Three surface-applied tracers were used. (1) A m m o n i u m nitrate fertiliser ("Nitram", 34.5% N; NH4, 17.2%; NO3, 17.3%) was placed on experimental plots at a rate equivalent to 350 kg ha -1 {120.7 N kg ha-l). (2) Potassium chloride, used at a rate equivalent to 700 kg ha -1. (3) lSN-labelled ammonium nitrate, with 5 atom % ~SN enrichment o f the NO3, applied at a rate equiv-

244

alent to 25 g m -2. For the lSN analyses, filtrates from extracted cores were freezer-stored prior to analyses. For all tracers, control plots were studied where no tracers were added, in order to evaluate background levels. This was an especially important step for unlabelled nitrate, in order to distinguish between leached nitrate and nitrate derived from mineralisation during rainfall events. Penetration depths were studied for individual rainfall events, with a range of rainfall a m o u n t s and intensities. Penetration depths were evaluated using 19-cm diameter soil cores taken to 70 cm soil depth. After extraction, bulk samples of 50 g soil were taken from across the whole core surface as the core was successively extruded. Micro-samples were also taken in a grid pattern across the face of the soil core, removing 1-cm 3 blocks from layers between the bulk sample layers, successively down the core (Smettern and Trudgill, 1983). The grid pattern provided a standard approach to micro-sampling, but it could have limitations o f spatial representativeness, given the probable spatial variation of the preferential flow process. Nevertheless, it provides an indicator of the presence o f preferential flow or o f uniform flow: if uniform flow occurs, the micro-sample data on tracer recovery should coincide with the bulk sample data for any one depth; departures o f the micro-sample data from the bulk sample data will indicate a variability of the flow process down the soil profile. Plastic dip-well tubes, 3 cm in diameter and perforated at the lower end~ were also inserted into auger holes in the soil profile to enable samples of freely draining soil water to be drawn up to the surface by hand p u m p for tracer analyses. Soil water outflow was monitored at the slope foot o u t p u t point, with flow being diverted into a weir pool via a sub-surface drain. Discharge was measured using a 90 ° " V " n o t c h weir plate and an " O t t " vertical R16 stage recorder. Discharge from the drain was sampled using a " R o c k and T a y l o r " 48 Interval automatic water sampler at 2-h intervals, reduced to 15 min for storm events. Samples were analysed within 1 day of collection. RESULTS

Preferential flow Methylene Blue dye staining o f a small soil block is shown in Fig. 1. Despite the weak structural organisation, as compared to a cracking clay soft, the non-uniform nature o f the flow is evident from the staining around the structures which do exist. There are stain-free areas in the centre o f m a n y peds and preferential, stained flow down the structural cracks visible between ped faces. For field cores which have been infiltrated with Methylene Blue and then sectioned into 2 ~ m thick slices, the areas of staining (Fig. 2) show t h a t there are evidently a limited number of entry points into the soil, with only 2.9% of the core area stained at 2 cm depth. Some edge staining of the core is

245 DYED WATER

JL

I

I

lcm []

METHYLENE BLUE STAINING

MAJOR STRUCTURAL CRACKS VISIBLE BETWEEN PED FACES

PARAFFIN WAX BLOCK

Fig. 1. Section through soil aggregate sealed in paraffin wax after throughput of methylene blue-dyed water.

apparent, despite lining the core with petroleum jelly, b u t the partial nature of the rapid flow is clear from all depths studied. There is also a trend o f increasing staining with depth, where bulk density decreases; much o f the staining is also linear, indicating the presence o f planar voids, presumably at ped faces, while other stains are circular, presumably cross-sections of tortuous biopores.

Penetration depths of surface-applied tracers Modal depths for the recovery of surface-applied nitrate and chloride in micro-samples were limited to the t o p 10 cm, as shown in the example of 27.3.83 in Fig. 3. Modal depths are, however, an inappropriate indicator o f surface-to-outputs linkages, and maximum penetration depths are the more relevant indicator o f such possible linkages. Bearing in mind the d e p t h of the soil profile at 70--85 cm, m a x i m u m tracer recovery depths were observed to be much deeper than modal depths, with a wide variety of recovery depth values. Maximum micro-sample recovery depths for chloride and nitrate were correlated, with an Rs (Spearmann Rank) value o f 0.84 (n = 13 cores), with cl~loride data ranging from 10 to 50 cm and nitrate from 20 to 50 cm. In detail, nitrate depths were b o t h higher and lower than chloride depths.

246 % STAINED AREA

DEPTH (cm)

2 (~

2.9

4 ~

2-3

6 ~

19.6

8 ~

67.5

10 ~

49.6

I

I

15cm

Fig. 2. Sections through a grassland soil core after the infiltration of water dyed with methylene blue. Departures from background levels were frequently small, however, indicating the importance o f less ambiguous tracing using lSN, as discussed below. The value o f the micro-sampling technique, as opposed to bulk sampling, is also shown, with m a x i m u m micro-sampling depths exceeding those shown by bulk sampling; in the case o f Fig. 3, bulk sampling showed a m a x i m u m recovery depth of only 30 cm. The interpretation o f this is that

247 0-

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MAXIMUM DEPTH

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SOIL MICRO SAMPLES

BULKED SOIL SAMPLES

6070-

Fig. 3. R e c o v e r y d e p t h s o f s u r f a c e - a p p l i e d n i t r a t e a n d c h l o r i d e f o r a r a i n f a l l e v e n t o f 9 n u n o n 2 7 . 3 . 8 3 ; i n t e n s i t y = 4 . 2 m m h -1, 6 = 0 . 4 1 c m 3 c m -s. F i v e v e r t i c a l s e c t i o n s a r e shown for micro-samples within the core (1--5) and 1 section for bulked samples across the core.

tracer penetration is p a t c h y and in preferential flow, and thus in bulk s a m p l e s the tracer is diluted b y non-labelled soil; in micro-samples, the patchiness o f tracer distribution is evident from micro-sample departure from bulk samples and the presence o f b o t h high and low recovery values within the core. Given the relationships b e t w e e n the occurrence o f preferential flow and intense rainfall, as reported in the literature, it is important to test the hypothesis that penetration d e p t h of tracer will increase with rainfall intensity, the envisaged mechanism being that the extent of preferential flow will increase as rainfall intensity increases. The data which test this hypothesis are shown in Fig. 4, with rainfall intensity plotted against the m a x i m u m recovery depths for tracers in soft cores taken from tracer plots after 12 storms with comparable antecedent soil moisture values. There are two data bodies. First, the curvilinear plot shown from 2.5 to 5.5 mm h -~ rainfall intensity, with'data falling close to a line which can be described b y the equation

248 6O E -i-

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Fig. 4. R e l a t i o n s h i p b e t w e e n rainfall i n t e n s i t y a n d m a x i m u m d e p t h o f t r a c e r recovery.

Dmax = 8.32 11"12 or

log Drnax = 4.79 + 10.2 log I

(1)

where I -- rainfall intensity (mm h -1 ) Dmax = m a x i m u m depth of tracer recovery in micro-samples (cm). Second, at high intensities, over 8 m m h -1, there were three data points at shallow depths which did not fit this relationship. There is no independent evidence to distinguish between the possible explanations for these data points. For this reason, it is seen as best to use the term "penetration d e p t h " to refer to the physical process of tracer movement, and the term "recovery d e p t h " to indicate the m a x i m u m depth o f tracer recovery by the microsampling technique. In the case of the three data points occurring above 8 m m h -1, recovery d e p t h is most probably an inadequate indication o f penetration depth. Apart from these data points, the hypothesis t h a t tracer penetration depth increases with rainfall intensity is not disproved by the recovery data shown on Fig. 4 for intensities between 2.5 and 5.5 cm h -1, there is also an indication t h a t the m a x i m u m d e p t h o f tracer recovery in micro-samples only exceeds modal recovery d e p t h o f 10 cm above a rainfall intensity of around 2.5 m m h -~. Plots o f recovery depths o f tracer against rainfall a m o u n t showed the same trend, but with a low correlation o f data (Rs = 0.24), indicating the lack o f value o f rainfall a m o u n t as a predictor o f recovery depths.

249 ~SN A t o m

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Fig. 5. P e n e t r a t i o n d e p t h s o f 15N tracer for 4 soil profiles, t a k e n as 4 vertical series o f micro-samples in o n e 1 9 ~ m diameter soil c o l u m n s u b s e q u e n t t o surface lSN application at 5 a t o m % and 20 m m rainfall.

Subsequent to the rainfall events studied with surface-applied nitrate fertiliser and chloride, as shown in Fig. 4, a rainfall event o f 20 mm was studied using surface-applied lSN. The data for a soil core taken after the application and rainfall event, with four vertical columns o f micro-samples (A--D), are shown in Fig. 5. Levels were found to be well above background, giving an unambiguous indication o f penetration depth. Penetration to the base o f the profile was seen in all cases (the lower sample being in stony regolith, which excluded the possibility of gaining an adequate sample in Section A). For the unlabeUed tracer data shown in Fig. 3, maximum penetration was to 50 cm; in the less ambiguous lSN labelled trace, the penetration was to 100 cm, showing that under intense rainfall, maximum penetration depths in preferential flow can be, in fact, to the base of the soil profile. Soil water outflow response Soil water outflow response to rainfall events is indicated by an increase in discharge at the outflow point. In addition, when there was no surface-

250

applied nitrate present in the drainage water catchment area, discharge increases were associated with decreases in nitrate concentrations in the soil drainage waters. Since no overland flow occurs in the study plot (the plots were designed to preclude overland flow from reaching the out-flow), the NO3 concentration decrease is taken as evidence of dilution o f the outflow by fresh input, surface
0

E

14/12/83

RAINFALL INTENSITY 6

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11

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251

A plot of soil drainage out-flow responses for 70 rainfall events is shown in relation to rainfall intensity and antecedent soil moisture (0, cm 3 cm -3) in Fig. 7. This plot is a test o f the second hypothesis that out-flow response is directly related to rainfall intensity, because o f linkage between the surface and output through preferential flow. Figure 7 shows the discharge responses,

401 35

30

25 20-

/

DISCHARGE RESPONSE (ml s"1 ) 0

No r e s p o n s e 0,1 - 5.0

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Fig. 7. Discharge response o f soil out-flow in relation to rainfall intensity and antecedent soil moisture at 5 - - 3 0 c m depth.

252 e v i d e n c e o f s u r f a c e - t o - o u t p u t linkage, and Fig. 8 s h o w s t h e nitrate d i l u t i o n r e s p o n s e s , e v i d e n c e o f rapid, preferential f l o w o f l o w nitrate, surface
401 35

30

25 / 20

MAGNITUDE OF DILUTION IN NITRATE CHEMOGRAPH (mg 1-1) O No r e s p o n s e •

0.1 - 1.0

•

1.1-3,0

•

>3.0

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rE

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O

O o

o8

8

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<.29

.29-.39

.40-.46

>.46

ea c m 3 c m "3

Fig. 8. Occurrence of dilution in the nitrate (NO3-N) ehemograph for soil out-flow in relation to rainfall intensity and antecedent soil moisture at 5--30 cm depth.

253 o f soil moisture at 2 bars, below which soil water is tightly held in the peds in an immobile state. The value o f 0 = 0.46 is the laboratory 0.46, out-flow responses were recorded, with three exceptions, independent of intensity -- and with out-flow even at some o f the very low intensities. This suggests t h a t surface-to-output transfer will normally take place in soils wetter than field capacity. At intermediate soil moisture values, 0 = 0.29--0.46, gravitational water would have drained off, but mobile matric water would be present. Under these conditions, with one exception, out-flow responses were recorded at rainfall intensities above 2.5 mm h -1. This indicates t h a t at the soil moisture values involved, an intensity threshold exists for out-flow response. It also implies that, if the pedal infiltration excess model applied, m a x i m u m pedal acceptance rates are also 2.5 mm h -1 for soil moisture values between 0.29 and 0.46 cm s cm -s. The field return value o f soil moisture, 0 = 0.39, is o f some value in differentiating the out-flow response--intensity relationship, as most outflow data points occur above 0.39, but there are five exceptions to this. This value probably represents a field moisture return involving b o t h evaporation/ transpiration and drainage; the laboratory drainage value o f field capacity only involved drainage. The hypothesis for the relationship between out-flow response and rainfall intensity is thus qualified by the inclusion o f antecedent soil moisture values, and the relationship is only valid for soil moisture conditions between the mobile:retained b o u n d a r y and laboratory determined field capacity. It should also be noted t h a t the threshold value for intensit y , 2.5 m m h -1, is close to the threshold identified on Fig. 4 for the departure of m a x i m u m tracer recovery d e p t h from the modal flow depth, again suggesting t h a t preferential flow exists over the threshold o f around 2.5 m m 5-1. The pattern o f soil out-flow dilution by preferential flow confirms this suggestion (Fig. 8) where the same thresholds and patterns are shown for nitrate dilution, with dilution occurring only above 0 = 0.29, above an intensity o f 2.5 m m h -1 at 0 = 0.29--0.46 (with one exception), and also at 0 > 0.46, irrespective o f intensity. The intensity threshold is difficult to predict for weakly structured soils. For strongly structured soils, peds can be isolated and the transmission rates o f the peds can be isolated in a simple permeameter (Bouma, 1980; Trudgill et al., 1983a, b; Smettem et al., 1983). Where peds are not so clearly defined, the procedure is more problematical. A t t e m p t s at ped isolation for

254

the study site soils gave pedal infiltration values o f 7.2--90 mm h -1 , well in excess o f the field values gained for out-flow response. These high values most probably relate to the existence o f structural pathways existing even within the small samples used (embedded in paraffin wax, see Fig. 1). In fact, minimum field infiltration values, gained using simple infiltrometers (Butt, 1978) of diameter 2.2 cm, give a range o f 1.2--3.0 mm h -z. This brackets the threshold value o f 2.5 mm h -1, but does not give a precise prediction o f it, most probably because of field variability o f flow patterns in the soil. Thus, prediction o f preferential flow thresholds by attempts to measure pedal infiltration values is liable to remain difficult for weakly structured soils.

Tracer recovery depth and out-flow response Soil out-flow response was recorded for a soil profile o f a depth o f 70--85 cm. Out-flow responses were frequently recorded when maximum tracer re-

RAINFALL INTENSITY

| E~

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RAINFALL AMOUNT

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,m, NITRATE CONCENTRATION~ IN SOIL OUTFLOW

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TIME (hr) Fig. 9. R e s p o n s e o f s o i l o u t - f l o w d e p t h s w e r e a s s h o w n o n Fig. 3.

for the rainfall event of 27.3.83

when

tracer recovery

255

covery depths were much less than this. For example, for the rainfall event o f 27.3.83 shown on Fig. 3, maximum recovery depths were 50 cm, b u t there was a clear out-flow response, which is shown on Fig. 9. Figure 9 shows b o t h a clear, rapid peaked response o f discharge and a clear nitrate dilution indicative of rapid preferential surface-to-output flow. This, again, is probably indicative o f the difference between recovery d e p t h and penetration depth. Indeed, when the available data on maximum d e p t h o f tracer recovery are plotted against discharge response {Fig. 10) and nitrate dilution {Fig. 11), the p o o r predictive value o f recovery d e p t h data is clearly shown. This is despite the good relationship b e t w e e n rainfall intensity and maximum recovery depths shown on Fig. 4. While the curve o f Fig. 4 would appear to be recording a real pattern, it is most probable that the sampling technique is inadequate for fully recording all the preferential flow paths which could contribute to out-flow. These pathways are liable to have a scattered distribution, and a more spatially extensive sampling programme would have to be involved before a complete picture o f preferential pathways and m a x i m u m penetration depths was gained. 80-

70-

60-

0 50-

40-

0

300

~2o-

0 0

"o .E

0 0

I 0 5 10 15 20 215 30 35 40 4~ ,50 55 Maximum depth of tracer recovery (cm) in soil cores

I 60

I 65

I 70

Fig. 10. R e l a t i o n s h i p b e t w e e n increase in discharge o f soil o u t - f l o w a n d m a x i m u m of tracer recovery for individual storms.

depth

256

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M a x i m u m depth of tracer recovery(cm) in soil cores

Fig. 11. R e l a t i o n s h i p b e t w e e n d i l u t i o n in t h e n i t r a t e ( N O 3 - N ) c h e m o g r a p h a n d m a x i m u m depth of tracer recovery for individual storms.

Omitting the step of the penetration depth--out-flow response relationship, and studying the rainfall characteristics--out-flow relationship, shows a useful possible approach. Both rainfall a m o u n t and rainfall intensity showed a positive relationship with solute response (Rs = 0.52 and 0.61, respectively). There was also a positive correlation between rainfall a m o u n t and intensity, but with a low Rs value (0.34). This, and the lower Rs value between a m o u n t and response (Rs = 0.52, above), focus attention again on the possible predictive value o f rainfall intensity (Rs = 0.61) w i t h o u t necessarily emphasising the need for detailed study of the intervening variable o f penetration depth, with its attendant problems of spatial sampling. CONCLUSIONS AND DISCUSSION The existence o f preferential flow has been clearly demonstrated using Methylene Blue, even on a weakly structured soil such as exists in the Slap-

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t o n catchment. The hypothesis that surface-applied solute penetration depth will increase with rainfall intensity is not contradicted for data at low rainfall intensities (2.5--5.5 mm h-l), b u t the sampling technique appears to be inadequate for indicating the full extent of penetration, especially at high intensities. For the second hypothesis, that out-flow response will be related to rainfall intensity, an intensity threshold of around 2.5 mm h -1 is indicated for out-flow response, in a similar fashion to intensity threshold for out-flow recorded b y TrudgiU et al. (1983a, b) at 3.6 mm h -1 and Smettem et al. (1983) at 0.5 mm h -1 for cracking clay soils. This threshold is, however, qualified for soil moisture conditions, and only applies in the range o f antecedent moisture 0 = 0.29--0.46. While preferential flow has been demonstrated, prediction of its occurrence using a pedal infiltration excess model is problematical since, in a weakly structured soil, the peds are difficult to isolate for the study o f their infiltration capacity; data on minimum field infiltration rates appear, however, to be useful in this context. Such minimum values will relate to soil surfaces where structural development and bioporosity will be minimal, and thus will supply useful information on m a x i m u m pedal acceptance rates. The data given in Figs. 7 and 8 give an indication o f the combinations of rainfall intensity and antecedent soil moisture where surface-applied solutes may suffer minimal losses in preferential flow. Predictably, above field capacity, surface application would result in losses, as even low-intensity rainfall events lead to out-flow responses. Below the mobile:retained boundary, losses in preferential flow would not be expected. Between these t w o conditions, only low-intensity rainfall events preclude loss, although the precise threshold value is difficult to predict for weakly structured soils. The lack o f soil water mobility in the weakly structured Slapton soils under dry antecedent conditions is in contrast to the existence o f by-passing flow in dry, cracking clay soils with well-developed peds, as shown b y Kneale and White (1984), White et al. (1983) and B o u m a et al. (1981). Kneale and White estimated that 10--20% of rainfall by-passed the t o p 0.09 m o f a dry, cracked clay--loam grassland soil at antecedent moistures o f 0.26--0.30 m s m -s (= cm 3 cm-S), with a by-passing threshold o f 2.2 mm h -~ rainfall intensity. This indicates the contrast b e t w e e n the more subtle nature o f preferential flow in a weakly structured soil and the more evident by-passing round distinctly developed peds in strongly structured cracking clay soils. It appears that in the weakly structured softs studied at Slapton, the dry soil aggregates are able to absorb moisture, even at the highest rainfall intensities, because o f the more diffuse nature o f the flow pathways; it is only at the high soil values, above the mobile:retained boundary, that the structural pathways operate preferentially. Thus, the existence o f the smaller structural peds in the Slapton soils appears to be crucial in absorbing rainfall at dry antecedent moistures; in dry, cracking clay soils where these small, absorbent peds are absent, by-passing flow round large structures and d o w n the large intervening voids will tend to be initiated more readily.

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On an operational level, it is suggested that preferential flow should be taken into account, even on weakly structured soils, if the benefits o f fertiliser application to the crop are to be maximised and the deleterious effects on drainage water quality are to be minimised. Preferential flow increases with rainfall intensity, but rainfall intensity is not always easy to predict from weather forecasts. However, it is clear that in weakly structured soils, soil moisture conditions can also provide a useful indicator of the probable occurrence of preferential flow. To evaluate the boundaries shown on Figs. 7 and 8, pressure-plate apparatus is needed to determine the mobile: retained boundary at 2 bars, although likely values for soils o f different texture classes can be indicated at a general level. Similarly, field capacity generalisations are available for different soil texture classes, b u t field capacity values can also be determined simply from soil columns. Such an approach could prove useful on a variety of similar soils. The difficulty remains, however, o f the accurate prediction o f rainfall intensity, and this is liable to prove to be the biggest obstacle to safeguarding drainage water quality and maximising agricultural production during fertiliser application. ACKNOWLEDGEMENTS

This work was supported b y the N E R C : G T 4 / 8 1 / A A P S / 4 2 . T o n y Thomas, Tim Burr, Brian Arkell and the Field Studies Council Staff at Slapton are thanked for their assistance and support, and J.J. Cocks and Sons of Low o r t h y Farm are thanked for access to, and use of, farmland. M. Johnson of A.R.C. L e t c o m b e is thanked for lSN analyses, with financial support from NERC: G R 3 / 5 1 2 9 .

REFERENCES Anderson, J.L. and Bouma, J., 1977. Water movement through pedal soils. 1. Saturated flow. Soil Sci. Soc. A m . J., 45: 413--418. Bouma, J., 1980. Field measurement of soil hydraulic properties characterising water movement through swelling clay soils.J. Hydrol., 45: 149--158. Bourna, J., Dekker, L.W. and Muilwijk, C.J., 1981. A field method for measuring shortcircuiting in clay soils. J. Hydrol., 52: 347--354. Butt, T.P., 1978. Three simple and low-cost instruments for the measurement of soil water properties. Hudderfield Polytechnic Department of Geography and Geology, Occasional Papers, 6. Butt, T.P., Butcher, D.P., Coles, N. and Thomas, A.D., 1983. The natural history of Slapt o n Ley Nature Reserve. XV: Hydrological processes in the Slapton Wood catchment. Field Stud., 5: 731--752. Kneale, W.R. and White, R.E., 1984. The movement of water through cores of a dry (cracked) clay--loam grassland topsoil. J. Hydrol., 67: 361--365. Smettem, K.R.J. and Trudgill, S.T., 1983. An evaluation of some fluorescent and nonfluorescent dyes in the identification of water transmission routes in soils. J. Soil Sci., 34: 45--56.

259 Smettem, K.R.J., Trudgill, S.T. and Pickles, A.M., 1983. Nitrate loss in soil drainage waters in relation to by-passing flow and discharge on an arable site. J. Soil Sci., 34: 499--509. Thomas, G.W. and Phillips, R.E., 1979. Consequences o f water movement in macropores. J. Environ. Qual., 8: 149--152. Troake, R.P., Troake, L.F. and Walling, D.E., 1976. Nitrate l o a d s o f S o u t h Devon streams. In: Agriculture and Water Quality. Tech. Bull., 32, M.A.F.F., H.M.S.O., pp. 340--351. TrudgUl, S.T., 1983. The natural history of Slapton Ley Nature Reserve. XVI: The soils at Slapton Wood. Field Stud., 5: 833--840. Trudgill, S.T., Pickles, A.M., Smettem, K.R.J. and Crabtree, R.W., 1983a. Soil water residence time and solute uptake. 1. Dye tracing and rainfall events. J. Hydrol., 60: 257-279.. Trudgill, S.T., Pickles, A.M. and Smettem, K.R.J., 1983b. Soil water residence time and solute uptake. 2. Dye tracing and preferential flow predictions. J. Hydrol., 62: 279-285. White, R.E., Wellings, S.R. and Bell, J.P., 1983. Seasonal variations in nitrate leaching in structured clay soils under mixed land use. Agric. Water Manage., 7: 391--410.