0038-075X/85/1392-0181$02.00/0SOIL SCIENCECopyright © 1985 by The Williams & Wilkins Co.

February 1985Vol. 139, No. 2

Printed in U.S.A.

LEACHING FROM A TROPICAL ANDEPT DURING BIG STORMS: ACOMPARISON OF THREE METHODS

A. E. RUSSELL, AND J. J. EWEL

We measured water and nutrient (Ca,Mg, K, and NO3) leaching in the field usingthree methods: (1) estimates of flux usingthe Darcy flow equation, (2) a water bal-ance, and (3) zero-tension lysimeters. Themethods were compared in an Andept nearTurrialba, Costa Rica, during two largestorms (33 and 22 cm wk"1), at two soildepths (25 and 110 cm), and in four kindsof 1- to 2.5-yr-old vegetation. The waterbalance was the most accurate method, be-cause the other two techniques sample onlyparts of the flow. Zero-tension lysimeterswere evaluated for use in measuring chan-neled flow under certain conditions.

Mean water flows calculated by the fluxand water balance methods were signifi-cantly different: 28.5 and 31.5 cm, respec-tively, at 110 cm during the 33-cm storm.The flux method overestimated water flowduring the 22-cm storm: 51.7 cm comparedwith 21.7 cm determined by the water bal-ance. Mean water flow determined fromzero-tension lysimeter data during bothstorms was substantially lower than thatestimated by the other two methods: 2.1cm for the 33-cm storm and 2.5 cm duringthe 22-cm storm.

Nutrient leaching was determined twoways. First, nutrient concentrations meas-ured using porous cup samplers were mul-tiplied by water flows derived from waterbalance data. Second, nutrient concentra-tions of leachate collected by zero-tensionlysimeters were multiplied by water flowsfrom the lysimeters. In general, nutrientlosses determined by the first method weregreater than those measured by zero-ten-sion lysimeters, but in some of the vegeta-tions, nutrient losses were so low that therewas no difference between the two meth-ods.

A few zero-tension lysimeters collectedlarge amounts of leachate even though thesoil did not reach saturation during thestorms. Solution collection in the lysime-

1 Department of Botany, University of Florida,Gainesville, Florida 32611.

Received for publication 5 June 1984; revised 7December 1984.

ters was mostly a large-storm phenome-non; of the solution caught annually by thelysimeters at 110 cm, 84% was collectedduring two, week-long storms. One lysim-eter (of 96 monitored) consistently col-lected leachate in excess of the rainfall andlow in nutrient concentrations. It is likelythat this lysimeter sampled channeledflow. In this soil, where lateral flow isrelatively unimportant and impeding lay-ers do not occur, the lysimeters provide ameasure of the occurrence and quality ofrapid channelized flow.

Over the last few decades soil scientists, hy-drologists, foresters, and ecologists have becomeincreasingly interested in monitoring downwardwater and nutrient fluxes in soils. However,problems with various methods for measuringthese fluxes, especially in field studies on smallplots, remain unresolved.

Methods based on soil physical principleshave provided valuable information concerningfluxes through plowed, homogeneous soils (Kel-ley et al. 1946; Richards 1949; Robins et al. 1954;Black et al. 1965; Hagan et al. 1967; So et al.1976; Libardi et al. 1980). Recently, applicationof these methods to unplowed and highly struc-tured soils has been questioned (DeVries andChow 1978; Bouma 1980; Beven and Germann1982).

Difficulties have also been encountered whena collection device is used to capture leachate(Cochran et al. 1970). In some studies, resultsobtained using the zero-tension lysimeter de-scribed by Jordan (1968) did not correspondwith those obtained by other methods, and thereasons for the discrepancies were unclear(Haines and Waide 1979; Haines et al. 1982).Downward water flow under unsaturated con-ditions cannot be measured accurately usingpan-type lysimeters, because capillary forces inthe soil surrounding the lysimeter may causesoil solution to pass around the soil directlyabove the lysimeter (Colman and Hamilton1947). However, nutrient concentrations of

181

182 RUSSELL AND EWEL

leachate collected by zero-tension lysimetershave been assumed to be representative of leach-ate in the soil profile (Jordan and Kline 1972).Despite the frequent use of zero-tension lysim-eters, it has not been determined just what thedevice actually measures under unsaturated soilconditions.

The objectives of this study were to estimatewater and nutrient flows during large storms,using the Darcy flow equation and zero-tensionlysimeters, and to compare these estimates witha water balance. Although there may be someoverlap in the water sampled by the two tech-niques, Darcy flow estimates the movement ofall water through the soil except that whichmoves freely through macropores or low-resist-ance channels. Most of the water captured byzero-tension lysimeters, however, is free-flow-ing, either because the soil is saturated or be-cause the water is flowing through a macroporeor channel.

MATERIALS AND METHODS

Study site

The study site is located near Turrialba, CostaRica, in the Florencia Norte forest of the CentraAgronomico Tropical de Investigacion y Ensen-anza (CATIE) at 9°53' N, 83°40' W. Turrialbais ~60 km from the Atlantic coast. The 2.4-hastudy site is situated at an altitude of ~650 m.

The warm, humid climate is classified as Afiin the Koppen system (Morrison and Leon1951). Tosi (1969) classified the area as tropicalpremontane wet forest sensu Holdridge (1967).According to CATIE's meteorological data,mean annual precipitation (1944 to 1979) is 264cm and the mean monthly temperature (1959 to1979) is 22.3°C.

The study site soil (of the Colorado series)has been most recently classified as a TypicDystrandept (Martini 1969; Harris et al. 1971).It overlies upper Miocene or lower Pliocene agebedrock (Hardy 1961). Through recent times,ash showers have contributed fresh ash to thissoil, which is derived from older lava. Physicalstructure of the soil is highly aggregated andstable (Hardy 1961). The soil is deep (> 6 m tobedrock) and freely drained. Biotic activity inthe soil is high. Animal tunnels, especially thoseof the leaf-cutter ant Atta cephalotes, extend toa depth of > 2 m (Alvarado et al. 1981). Thesoil, like many Andepts, has unusual physical

properties, including a high water-holding ca-pacity; at tensions as high as ~15 bar, the mois-ture content by volume is ~30%. Soil losses byerosion are low (Ives 1951).

In early 1979 the second-growth vegetation atthe study site was felled and burned (Ewel et al.1981). Immediately after the burn, four kinds ofvegetation were initiated as part of a broadresearch project intended to explore the feasi-bility of using complex vegetation as models fortropical agroecosystem design. The experimen-tal vegetation types (described in Ewel et al.1982 and Blanton and Ewel 1985) consisted ofthe successional vegetation that regenerated fol-lowing the burn, plus three experimental ecosys-tems that ranged from a 1-yr-old monocultureof trees (Cordia alliodora) to three 2.5-yr-oldcommunities that contained > 50 species oneach plot. Mean canopy height in the vegeta-tions ranged from 3.9 to 5.2 m, and the tallesttree at the site was 13.0 m. Six plots of each ofthe four kinds of vegetation were established;each of the 24 study plots was 14 m on a side(196 m2), plus a 1-m-wide buffer strip.

Water flow

Water flows were determined during two largestorms and at two depths, 25 and 110 cm. Thefirst storm, of 32.62 cm, occurred during theweek of 7-13 November 1981. The second storm,of 22.24 cm, took place from 22-27 November1981. Soil water flows during the two largestorms were calculated by three methods: (1)Darcy flow (referred to hereafter as flux), (2)water balance (referred to as balance), and (3)zero-tension lysimetry (referred to as lysimetry).The first two methods involved measurementsof soil moisture tension.

Mercury manometer tensiometers, describedby Richards (1949) and Slavik (1974), were usedto measure soil moisture tension (also known aspressure potential, matric potential, suction,pressure deficiency, and capillary tension). Onetensiometer was installed at each of four depths,15, 45, 75, and 115 cm, in each of the plots.During installation we attempted to minimizesoil compaction and smearing around the cup,while maintaining good soil contact. We ex-cluded microsites that obviously received chan-neled flow (e.g., at the bases of trees and withinant tunnels). Insofar as possible, the 96 tensiom-eters were read at the beginning and end of eachof the several rain events that comprised the

LEACHING DURING BIG STORMS 183

week-long storms: 15 times during the 33-cmstorm and 7 times during the 22-cm storm.

By the flux method, a water flux (q) wascalculated for each study plot at the beginningand end of each rain event during the twostorms. The fluxes, integrated over the stormtimes, estimated the downward water flow dur-ing the storm. Fluxes were calculated using Dar-cy's law

q = K(d) dH/dz

where K is the hydraulic conductivity at thevolumetric content 6, and dH/dz is the rate ofchange in total hydraulic potential (H) withrespect to depth z. Two sets of relationshipswere determined experimentally to calculate aflux: (1) soil moisture retention, the relationshipbetween soil moisture tension (measured usingtensiometers) and volumetric water content; and(2) the relationship between volumetric watercontent and hydraulic conductivity. To deter-mine soil moisture retention, one intact samplecore, 3.5 cm in diameter and 1 cm deep, wastaken from three of the replicates of each of thefour types of vegetation, at four depth intervals(0 to 15, 15 to 45, 45 to 75, and 75 to 115 cm),and for each of the seven soil moisture tensionsfor which a gravimetric water content was to bemeasured, yielding a total of 336 samples. Pres-sure-plate and pressure-membrane apparatusesproduced the seven tensions used to create themoisture retention curves: 0.01, 0.05, 0.10, 0.20,0.33, 0.50, and 1.00 bar.

Hydraulic conductivity was measured in situin the 11-yr-old second-growth forest betweenstudy plots, using the internal drainage methodreviewed by Hillel (1980). A 1.4-m-diameter soilcore, trenched to a depth of 120 cm and encasedin plastic, was brought to saturation. Soil mois-ture was monitored as a function of time aswater drained from the core, using eight ten-siometers (two sets at four depths: 15, 45, 75,and 115 cm).

Because this study concerned water flow dur-ing large storms, three assumptions were madethat simplified calculation of a water balance.The first was that, during large storms, theproportion of rainfall intercepted by the vege-tation was small relative to the total rainfall andcould be ignored. The second assumption wasthat all water flow was vertical and downwardthrough the profile. Overland flow of water was

not observed on the study plots during thestorms. Lateral flow along the interface betweenhorizons was not investigated, but was assumednot to occur because a hardpan or other imped-ing layer does not occur at the study site. Third,water losses via evapotranspiration were as-sumed to be small and could be ignored. There-fore, a water balance could be calculated as thedifference between total rainfall and the changein soil water storage resulting from the rainfall.Amount and timing of rainfall were measuredusing a tipping-bucket rain gauge that recordedrain in 0.25-mm increments. As in the fluxmethods, the measurements of soil moisture ten-sion before and after the individual rain eventsduring the storms were converted to a volumet-ric water basis using the experimentally derivedsoil moisture retention curves.

In the lysimetry method, the volume of leach-ate collected as outflow from zero-tension lysim-eters was measured daily during the two storms.The apparatus, described by Jordan (1968), is astainless-steel trough, 5 cm wide, 30 cm long,and 4 cm deep. The top is fitted with a fiberglassscreen that supports glass wool. Stainless-steeldrain rods and tubes are situated beneath thescreen. A pit 1.5 m deep was dug in the topo-graphically lowest quadrant of each of the 24study plots. On the upslope side of the pit, atunnel ~1 m long was excavated such that itsloped slightly upward from the wall of the pit.The lysimeter was inserted in the tunnel andpressed upward as the space below the troughwas backfilled. Tygon tubing was used to con-nect the outflow tube on the downslope side ofthe lysimeter to a vented collection bottle. Twosets of lysimeters were installed at each of twodepths, 25 and 110 cm, in each of the 24 plots.

Nutrient flows

Nutrient flux was calculated by multiplyingthe water flux by the nutrient concentration ofthe soil solution (Hillel 1980). Soil solution con-centrations of the following nutrients were de-termined: Ca, Mg, K, and NO3. Soil solution wassampled two ways and at two depths (25 and110 cm) in all 24 study plots.

For the flux and balance methods, soil solu-tion nutrient concentrations were determinedusing large-diameter (4.8 cm) porous cup sam-plers. They were first described by Briggs and

184 RUSSELL AND EWEL

McCall (1904) and recently evaluated by Han-sen and Harris (1975) and Silkworth and Grigal(1981). Two sets of porous cup samplers wereinstalled at two depths, 25 and 110 cm, in eachof the 24 plots. Samples were taken once duringthe 33-cm storm, over a 48-h period at a tensionof 600 mbar. Samples were also routinely takenat the study site once a month; thus nutrientconcentrations over time and under differentsoil moisture conditions were monitored. Sam-ples from the two samplers at the same depthwithin a study plot were pooled.

The leachate collected from the zero-tensionlysimeters was also analyzed for nutrients. Twosets of samples, one for each storm, were col-lected. Samples were collected daily, but werepooled for chemical analyses over the entireweek-long storm. Samples were filtered throughno. 40 Whatman paper, preserved with boricacid if destined for NO3 analysis, and refriger-ated until analyzed. Concentrations of cationswere determined by atomic absorption. For N03

analysis, AgSO4 was added to eliminate interfer-ence by Cl~ ions; samples were analyzed with anN03-sensitive electrode.

RESULTS

Water flows

Hydraulic conductivity, as measured in situwith the giant soil core, varied linearly over arange of volumetric moisture contents fromabout 58% to saturation (76%). This corre-sponds to a range of soil moisture tensions of0.01 to 0.00 bar. At a soil depth of 15 cm, itranged from 14.0 to 42.7 cm h"1. The infiltrationrate, which was measured as water was suppliedto the core, was 35.4 cm h"1 at saturation.

Mean (standard error, SE) soil moisture ten-sions, averaged over all plots and depths, werehighest prior to the onset of storms: 0.112(0.008) bar before the 33-cm storm and 0.105(0.005) bar before the 22-cm storm. When thesoil was wettest, mean (SE) soil moisture ten-sions were 0.025 (0.002) and 0.028 (0.002) barduring the 33- and 22-cm storms.

Mean hydraulic gradients at the 110-cm depthprior to the onset of the 33- and 22-cm stormswere -0.60 (0.14) and -0.76 (0.10), respectively.Even when the soil was wettest during the twostorms, mean (SE) hydraulic gradients were

-0.70 (0.06) and -0.62 (0.07), respectively, in-dicating that flow was not steady state but tran-sient.

Two sets of ANOVAs were performed to testfor differences in (1) water flows between thetwo tensiometric methods, and (2) water andnutrient flows in the balance and lysimetrymethods. In both cases, ANOVAs were carriedout separately for each storm, and the modelincluded terms for the depth at which the meas-urement was made, the type of vegetation, andall possible interactions.

Water flows determined by the two tensiom-etric methods were significantly different atboth depths and during both storms (p > F =0.001). During the 33-cm storm, mean waterflows calculated by the flux and balance methodswere 28.5 and 31.5 cm, respectively, at 110 cm,and 7.1 and 28.4 at 25 cm. During the 22-cmstorm, the difference between the two meanswas more striking: 51.7 cm determined by theflux method compared with 21.7 cm calculatedby the balance method at 110 cm, and 21.9 and20.9 cm, respectively, at 25 cm. Mean fluxes pastthe two depths were not different as determinedby the balance method, but were different ascalculated by the flux method. The flux methodpredicted a high frequency of near-zero valuesand a broader range of water flows, whereas thebalance method predicted means that closelyapproximated the rainfall received (Fig. 1). Be-cause the flux method predicted some unlikelywater flows, only balance method water flowswere used to calculate nutrient flows. Lysimetersat 110 cm captured only 2.1 cm of water duringthe 33-cm storm and 2.5 cm during the 22-cmstorm. There were no significant differences inwater flows into lysimeters past the two depthsor among the four vegetations.

Nutrient flows

Mean nutrient losses determined by waterbalance/porous cup sampler data were substan-tially greater than those determined by lysime-try (Table 1). However, due to the influence ofvegetation on soil-solution nutrient concentra-tions (especially solution sampled using porouscups) and high variability among samples, nu-trient losses estimated by the two methods didnot always differ significantly, even though theyprobably reflect different phenomena.

LEACHING DURING BIG STORMS 185

oCl-

IO 20 30 40 50 60 70 80 90 100

LU

OUJcnLu

RAIN =22.2 cm

^ BALANCE METHOD

3 FLUX METHOD

40 50 60

" LL)Oo:LU

I- 40 Q_

-p'/r-160 220

WATER FLOW CLASS ( c m - w k " 1 )

FIG. 1. Water flow calculated by two methods.

TABLE 1

Comparison of methods of determining water and nutrient flow

Depth, cm

25

110

Storm size, cm Method

33 Balance

Lysimetry22 Balance

Lysimetry

33 Balance

Lysimetry22 Balance

Lysimetry

Water0

28.36(24.72-30.53)*

2.4320.89

(19.75-22.79)1.13

31.52(26.95-37.26)

2.1021.68

(19.40-26.58)2.53

Ca

3.78(2.29*5.03)

0.272.98

(1.55*4.23)0.08

3.46(1.02*6.46)

0.232.41

(0.80*3.80)0.04

Mg

2.10(1.44*3.24)

0.111.57

(0.94*2.40)0.04

1.71(0.76*3.24)

0.041.21

(0.58*2.43)0.03

K

3.16(0.52*7.26)

0.202.19

(0.33*4.98)0.09

1.34(0.34*3.58)

0.010.99

(0.23*2.86)0.06

N03

4.14(0.83*7.48)

0.193.20

(0.46*5.53)0.07

3.24(0.20*7.20)

0.312.27

(0.15*4.28)0.06

b Parenthetical values are ranges of means corresponding to different vegetation types. Means separated by- did not differ significantly; means separated by * indicate that there were significant differences amongvegetation types. None of the lysimetry data differed significantly among vegetation types.

186 RUSSELL AND EWEL

DISCUSSION

Balance method

The water balance method consistentlyyielded water flux estimates that closely approx-imated rainfall. Therefore, estimates obtainedby this method were used to evaluate flows de-termined by the other two methods.

Flux method

The flux method predicted some unlikely wa-ter fluxes. The most obvious explanation in sucha heterogeneous environment is that the samplesize was too small to enable us to accuratelyestimate the true mean. By applying Stein'sprocedure for determining appropriate samplesize (Steel and Torrie 1960) to our data from all24 plots, we calculated the sample size requiredto estimate the true mean to within 5 cm ofwater flow at a 95% confidence level. For waterflows past 110 cm, a sample size of 1093 wouldhave been necessary when using the fluxmethod, compared with only 10 for the balancemethod. Thus, the flux method may have yieldedreasonable estimates for one storm or at onedepth but not another due to chance alone.

Calculating fluxes based on (1) soil moisturetensions averaged over depth as well as overtime and (2) our lowest estimates of hydraulicconductivity (the equations for the 15 cm depth)did not yield a more realistic flux for the 22-cmstorm. A more reasonable mean water flow (past

110 cm) of 28 cm for that storm is obtained ifthree plots with extremely high fluxes are ex-cluded from the calculations. However, therewas no reason to believe that the data fromthese plots were invalid. The microsites wherethese tensiometers were located were consis-tently wetter during the 22-cm storm than dur-ing the 33-cm storm, perhaps due to subtlechanges in vegetation structure or below-groundwater-flow patterns. These data emphasize thatfluxes can be highly variable both spatially andtemporally.

Lysimetry study

Lysimeters probably trap only a small fractionof the water that leaches through the soil duringlarge storms. A closer look at the data showsthat most of the lysimeters collected little or nowater, but a few collected large amounts (Fig. 2)and accounted for most of the water collectedby all lysimeters. There was a good correlationbetween the two large storms in amounts ofwater collected by individual lysimeters (r2 =0.99).

One might have expected that none of thelysimeters would have collected any leachate,because two independent sets of data indicatedthat most of the soil matrix did not saturateduring the storms. First, soil moisture tension,as determined by > 90 tensiometers, did notreach zero. Second, the infiltration rate of thesoil is high (35.4 cm h"1 at saturation) and farexceeds the highest rainfall intensity (2.1 cm h"1

so-"

20 -

CC

E-sens- n -

DEPTH- 25cmRAIN = 33cm

jLnEfcfe, f - — ̂ -/fli

-60 3°-

-

-40 20"-

-20 1 0 -

- 0 0-

DEPTHsRAIN-3

t_ ;

od

DEPTH=25cmjj RAIN-22cm

iyl . . — n ay/_

-60

•40

- 20 1i oHI

1 .

r aoDEPTH. 110cmRAIN. 22cm -60

- 4 0

-20

-i rr//^-/Ar-t 010 15 "28 0 5 10

WATER FLOW CLASS (cm-wk" 1 )

FIG. 2. Frequency distribution of water flow as determined by zero-tension lysimeters.

LEACHING DURING BIG STORMS 187

during the 33-cm storm). Thus, the results bringup an interesting question: How did water getinto any of the zero-tension lysimeters if the soilmatrix was unsaturated? One possibility is thatless permeable layers at any point above thezero-tension lysimeter would cause water arriv-ing via unsaturated flow to pond up until suffi-cient pressure built up and overcame the soil-air interface at the soil-lysimeter junction. How-ever, this does not explain how one lysimetercollected an amount that exceeded the rainfall(58.7 cm during the 33-cm storm and 50.7 duringthe 22-cm storm). Most of the flow from thatlysimeter occurred on 11 and 12 November whendaily rainfalls were 10.0 and 2.8 cm, respectively.Clearly, that lysimeter was collecting water fun-neled from an area larger than that interceptedby the lysimeter. The question of how rainfallcould be channeled was not addressed in thisstudy; however, some field observations, plusresults of other researchers provide insight intothe problem.

The face of one of the soil pits intersected achannel ~3 cm in diameter, at a depth of ~40cm. Faucetlike water flow from the channel wassteady and rapid during peak rainfall; the 3.6 m3

soil pit filled with clear water within 1 h, whichwould have required a flow rate of at least 60 000cm3 min"1. This demonstrated the existence ofpreferential pathways of water flow at the studysite and their capability of conducting largeamounts of water.

Over 100 yr ago, Lawes et al. (1882) discoveredthat water added to soil profiles can move im-mediately through open channels and interactonly slightly with water in the soil matrix. Morerecent work has verified that channels formedby roots and animals can be quite effective inconducting water through soils, even when thesoil is unsaturated (Aubertin 1971; Beasley 1976;Quisenberry and Phillips 1976; Mosley 1979,1982). Bouma et al. (1982) monitored the volumeof water that had to be applied to individualchannels to keep them filled. For continuous,more or less vertical worm channels < 6 mm indiameter and extending to a maximum depth of1.6 m, they measured infiltration rates of 140 ±30 cm3 min"1. Flow rates into mole burrowsextending to a depth of 50 cm were 400 ± 100cm3 min"1.

Collection of leachate in the lysimeters at adepth of 110 cm is clearly a large-storm phenom-enon. Data taken during the two, week-long

storms were compared with the data taken dur-ing the 50 wk of the year when large storms didnot occur. Of the water collected by lysimetersat 110 cm over a 1-yr period, 84% was collectedduring the two storms. Nutrient concentrations(excepting K) were especially high in the leach-ate captured during the first storm, which ac-counted for 86% of the Ca, 67% of the Mg, and77% of the NO3 caught during the entire yearby lysimeters at 110 cm. The second stormyielded leachate at 110 cm that was much lowerin nutrient concentration; it accounted for only5, 15, and 13% of the annual amounts of Ca,Mg, and NO3 picked up by lysimeters at 110 cm.

There is further evidence that these lysime-ters sample different flow than do porous cupsamplers. In general, cation concentrations ofsoil solution collected by the two apparatuseswere different. Although there was no consistenttrend in nutrient concentrations with respect towater flow rate from the lysimeters, the leachatefrom the lysimeters that collected amounts ofwater in excess of rainfall tended to have ex-tremely low nutrient concentrations (Table 2).In particular, the leachate from the lysimeterthat yielded flow rates of > 50 cm wk"1 duringthe storms had consistently low nutrient con-centrations (mg LT1): 0.05 to 0.09 Ca; 0.05 to0.09 Mg; 0 to 0.05 K; and 0.11 to 0.25 NO3.These concentrations were almost as low asthose of the incoming rainwater at the site (Hen-dry et al. 1984).

TABLE 2Nutrient concentrations of water sampled by

lysimeters and porous cups during large storms

Nutrient

Ca

Mg

K

N03

Lysimeter catches"(mg L-')

Volumes> rain

0.30(0.15)0.13

(0.07)0.10

(0.10)0.14

(0.11)

Volumes< rain

2.06(0.46)0.56

(0.07)1.29

(0.56)2.16

(0.59)

Porous cupsamplers

1.27(0.21)0.64

(0.07)0.81

(0.26)1.16

(0.28)

"For both lysimeter catches and porous cup sam-plers, means (standard errors) are based on data fromboth depths (25 and 110 cm) and all four types ofvegetation during the 33-cm storm.

188 RUSSELL AND EWEL

It is likely that under unsaturated conditionsin soils with equally permeable layers, zero-tension lysimeters measure channeled flow. Cer-tainly this must be true for lysimeters that col-lect water in excess of the incoming rain.Whether this is the case for lysimeters thatcollect smaller amounts of rain could be dem-onstrated only by verifying the existence ofchannels in the soil or saturated microsites andby ensuring that the installation procedure didnot cause local ponding above the lysimeters.Where lateral flow is negligible and impedingsoil layers do not exist, the zero-tension lysim-eter may provide a measure of the occurrenceand quality of rapid channelized flow under un-saturated soil conditions.

ACKNOWLEDGMENTS

This study was part of a cooperative researchproject between CATIE and the University ofFlorida. It was supported by NSF grant DEB80-11136 and by a University of Florida Grad-uate School Fellowship. We thank C. W. Berishand N. Price, for installing the lysimeters, andJ. Bouma, J. A. Cornell, R. F. Fisher, P. S. C.Rao, W. Schlesinger, E. L. Stone, P. Vitousek,and anonymous reviewers for helpful sugges-tions.

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