lnt. Agrophysics,1993, 7,133-140 MEASUREMENT TIME AND SPATIAL VARIABILITY OF FIELD INFIL1RATION M. Kutilek, V. Kuraz, M. Krejca Department of Irrigation and Drainage, Czech Technical University in Prague Thaikurova 7, 166 29 Prague 6, Czech Republic A b s t r a c t. Infiltration studies were performed on For the estimation of some soil hydraulic arenic chemozems of quartemality fluvial terrace of Labe functions the inverse solution of inftltration is (Elbe). We studied the applicability of infiltration equations of Philip (2-pararnetric ), and 3-pararnetric, of Brutsaert and applicable as an expedient and fast method. Hy­ Swartzendruber, using 70 inflitration tests performed on a draulic functions and parameters of inftltration regular grid on a plot covered for 4 years by grass. The best are then used for the quantitative discussion on applicability was found for the 3-pararneter equations, but the dynamics of soil structure. Our studies there is no unique recommendation. The probability density function (PDF) of estimates of soil hydraulic characteristics were, therefore, aimed at the evaluation of the is defonned by errors of estimates due to the approximate field inftltration tests. The main problems were: character of equations used. The log-normal distribution is a 1. Field testing of quasi-analytical and ap­ well-acceptable approximation for sorptivity S, saturated proximative equations of infiltration. hydraulic conductivity Ks and rates of inflitration. Only for 2. Application of the tested equations to the some estimates of hydraulic functions a weak spatial va­ riability was found for the distance of 7.5 m. Long time va­ study on the long time alteration of the soil fabric riability of infiltration and its parameters after ploughing and porous system after ploughing. and subsequent grass planting was studied on the other plot. The space variability is restricted to a pedo­ There was a significant drop of Ks just after one season of logically homogeneous district. This heteroge­ grass growth and harvests. Low soil structural stability was indicated by the infiltration tests. neity can be described as stochastic within the Key w o rd s: field infiltration, arenic chemozems, in­ frame of the deterministic homogeneity of the filtration equations pedotop. METHODS IN1RODUCTION Site description and methods of mea­ All transport phenomena, the flow of water surement in soil and the accumulation, transformation and transport of the actual and potential pollu­ The experimental station is situated at tants, as well as gas diffusion, are related to the Ti~ice, district M~lnik, in Central Bohemia soil porous media for given boundary condi­ (abo'ut 20 km north-west from Prague). The tions. The quality of these media is dependent total area of about 0.5 ha is divided into 8 plots, upon the structural development of soils. The each of 52 x 68 m. The experimental plots lie on primary and secondary aggregation is, e.g., re­ a quatemaly fluvial terrace of gravel sand, over­ flected by the soil water retention curve and by layed by permeable sand. The landscape is the unsaturated conductivity function [7]; the practicaly a plane, with slight irregular undula­ nature of the structure is links to the type of the tions. model of the soil porous system [6]. Climatologically, the area belongs to the 134 M. KUTILEK et al. wann region, dry with moderate winter. The Infiltration was measured in September at total mean annual precipitation is 518.8 mm, initial soil water content 9F0.222 up to 0.236, precipitation during vegetation season (from at the depth 15-20 cm, and it was practically April to November) - 343 mm. The mean an­ constant during the time period of measuring, nual temperature is 8.6 oc (vegetation season with slight space variation only. In addition to 14.8°C). infiltration tests, saturated hydraulic conducti­ The whole experimental area is covered by vity K, was estimated by two further methods. arenic chemozems of the carbonate variety K was then evaluated by the following meth- (FAO). The thickness of horizons and subhori­ ' ods: zons is variable due to the fluvial influence upon a) Ponded infLltration by Brutsaert's [1] equa­ the otherwise homogeneous sands of the parent tion (see later). material of the C horizon. b) Laboratory estimates by falling head per­ The sandy-loamy topsoil with about 2.5 % of meameter with hydraulic gradient « 1, vol­ humus and 15 % of particles under 0.002 mm 3 ume of core samples was 100 cm , samples reaches down to the depth of 40-80 cm. Below were taken from a layer of 5-10 cm below it is loamy sand with some loamy lenses (at surface. about 60-100 cm) and permeable coarse sands c) Measurements with Guelph permeameter in (below 100 cm). The ground water table is about shallow boreholes of 15 cm in depth and 8 4-5 m below the surface. cm in diameter. The water level in the bo­ Two experimental plots have been chosen: rehole was kept at 10 cm above the bottom. Plot A was covered by grass (DactyUs glomera­ The evaluation followed the method of la), where grass had been grown during preced­ Reynolds and Elrick [ 11]. ing 4 years. On plot B, the time variability of On plot 8 , repeated series of 20 infiltration infiltration characteristics was studied. The frrst tests were performed. A rectangular net with a set of infiltration tests was performed just after grid of 14 m was used in order to exclude totally ploughing and basic agricultural cultivation be­ the spatial dependence and thus the deforma­ fore sowing of grass in April. The second set of tion of variance. At the frrst series in the spring, infiltrations was realized in autumn, after 5 the surface was without wegetation, the soil months of growth of grass and harvesting. The was very loose after agrotechnical cultivation. third set of infLltrations was in spring the fol­ The second series was done in autumn on the lowing year. plot already covered by grass, the third series The spatial arrangement of tests in plots A was similar, but in the spring the following year. and B differed slightly, mainly in the number of Vegetation cover was regularly harvested and it tests. was without irrigation. The duration of infiltra­ On plot A, a series of 70 infLltration tests tion tests was chosen to be 1.800 s due to the was performed at equidistant positions. The dis­ sharp boundary between the cultivated and non tance between the centers of the infLltration cultivated part of Ah horizon. There, the flow rings was 7.5 m, the rectangular mesh was paths of water diverged extremely when the formed by 7 columns and 10 rows. Duration of wetting front reached the boundary. The time of the tests was 3.600 s each to avoid the irregu­ 1.800 s was short enough to prevent this effect. larities earlier observed in long time infLltration. Otherwise," infiltration tests were performed in They were caused mainly by the divergence of the same way as on plot A. flowpaths on the transition between Aph hori­ zon with the root system and the subhorizon Evaluation of inftltration tests Ah(c) without distinct root system and with the increase of clay particles at some instances. The During the infiltration tests, the cumulative diameter of the inner ring was 37.5 cm, and that infiltration I (cm) was read in timet (min). Ex­ of the outer ring- 60 cm. The positive head of 2 perimental data were fitted to the following in­ cm ±0.5 cm was held in both rings. filtration equations which belong to the subclass TIME AND SPATIAL VARIABIU1Y OF INFILTRATION 135 of approximative equations with physical inter­ (5) pretation of parameters. Philip·s [9] algebraic two-parameter equa­ The extension of (1) to the three-parameter tion is: Eq. (3) reduced substantially the truncation er­ 1 = sp t 1.1 +At (1) rors in the first two terms; the estimation of S and K, is theoretically very good with errors the inflltration rate v = dl/dt is: below 10 %. The same is true for the next two 1 (2) 3-parameters equations. v=-S tVl+A 2 p In order to validate Philip's time series equ­ ation tot --+oo, Swartzendruber [12] proposed SP (L·T-Vi) is the approximation of sorptivity S the exponential form of l(t) and, truncating all which is the first term in time series solution terms in exponential time series except the first of Philip [8]. Owing to the truncation of infi­ one, he proposed: nite series solution, the truncation error eP can (6) be neglected and SP is then considered as a good estimate of sorptivity [5]. Parameter 1 A (LT- ) is (A2 + K; + e") where A2 is the sec­ where Ao (r-1.-'l) depends upon hydraulic func­ ond term in the time series solution, K; is the tions of the soil and it includes the truncation unsaturated conductivity at initial water con­ error, too, similarly to Swartzendruber·s esti­ tent e,, £" is the truncation error, £" > £p. It is mate of saturated conductivity (K,\ t. Sub­ A = m K,, m stituting 4 K, /3 S, for A we obta'in i two-pa­ supposed that where the value of 0 is in the range between 0.2 and 0.67 [10]. De­ rameter equation. On the other hand, if we tailed studies show that m depends upon 6; and consider only the first four terms of a series ex­ pressing exp ( -A tVl) we obtain an equation t, and sometimes exceeds a theoretical upper 0 limit of 2{3. The error of estimate of K, identical to Eq.