J. agric. Engng Res. (1990) 45, 69-76

The Rewetting of Partially Dried Grass Swaths by Rain: Part 2, Exploratory Experiments into Absorption and Drying

Rates

M. B. McGECHAN,* R. E. Pr r r t

Some exploratory experiments are described to estimate the time constants for absorption of surface moisture by plant tissue, and for runoff of surface moisture, when partially dried grass swaths are rewet by rain in the field. Absorption was measured in small grass samples which were rewetted while drying on a laboratory thin layer drying rig. Runoff was measured from sections of full size swath suspended from a basket on a balance. Estimated time constants in exponential relationships were 15 h for re-absorption and 3-5 min for runoff.

1. Introduction

In Part 1 of this paper, Pitt and McGechan 1 describe the development of alternative models of the process of rewetting of grass swaths by rain. However , they demonstrated a lack of knowledge about appropriate time constants, or resistance coefficients, to represent the rate at which surface water is absorbed into plant tissue and the rate of runoff of surface moisture. This paper describes small scale exploratory experiments to obtain estimates of both these time constants, exploiting equipment already being used at the Scottish Centre of Agricultural Engineering (SCAE) for swath drying experiments.

2. Investigation into absorption rate

2.1. Materials and methods

An essential part of each experiment on swath drying in the field or wind tunnel 2"3 is a parallel experiment in which samples of the same grass are dried in thin layers, at a constant temperature of 25°C and relative humidity of 50% (e.g. Spencer et al.4"5). The experimental equipment and analytical techniques are based on that developed by Hale. s Six grass samples from each field t reatment are normally dried in this way; the equipment can take up to 60 samples, allowing up to 10 field treatments.

Laboratory experiments into the absorption rate (dMa/dt) were carried out in 1987 in parallel with the normal thin layer drying tests for the field or wind tunnel experiments. In each case, 12 samples of perennial ryegrass were subjected to periodic rewetting treatments and 12 similar samples were dried undisturbed on the thin layer rig in the normal way. The rewetting t reatment consisted of removing the tray from the thin layer drying rig, adding water with a small hand-held garden sprayer until it began to drip through the bottom, then placing the tray in a polythene bag for storage for different times. About 100g of water was added to a sample, each of which had a dry mass in the region of 80-100 g. At the end of the storage period, the sample tray was returned to the thin layer drying rig. Trays were weighed immediately before and after rewetting, and after removal

* Scottish Centre of Agricultural Engineering, Bush Estate, Penicuik, Midlothian EH26 OPH, Scotland t Department of Agricultural and Biological Engineering, Cornell University, Ithaca, NY 14853-5701, USA

Received 8 September 1988; accepted in revised form 29 August 1989

69

0021-8634/90/010069 + 08 $03.00/0 (~) 1990 The British Society for Research in Agricultural Engineering

70 R E W E T T I N G OF GRASS SWATHS, 2

Notation

M dry basis moisture P~-6 parameters of exponential content, fraction relationship fitted to

Ma absorbed moisture content runoff experimental data (including tissue moisture) t time, h

M a . . . . maximum absorbed ~ resistance coefficient for moisture content runoff of surface moisture,

M0 initial dry basis moisture h content T.d resistance coefficient for

M r . . . . maximum loosely held absorption of surface surface moisture content moisture into plant tissue,

Ms . . . . maximum adhered surface h moisture content

from storage. Moisture adhering to the outside of the tray and to its gauze bottom was wiped off before weighing. The trays were also weighed after returning to the thin layer drying rig, frequently at first as surface moisture evaporated, less frequently later. At the end of each experiment the sample trays were dried in an oven for 24h at 103°C, weighed, emptied and weighed again along with all the normal thin layer sample trays. The balance on which the weighings were carried out was connected to a microcomputer, so all weights were recorded automatically on a cassette tape, and later entered into a mainframe computer.

The experimental design based on 12 samples consisted of three storage periods, (4 h, I h and zero), two field treatments (cut with a steel spoke mower conditioner and cut by hand, as with the non-rewetted samples) and two replications. In the first experiment (cut 17 June 1987), rewetting was carried out three times, about 1, 20 and 44 h after cutting. In the second experiment (cut 25 June 1987) rewetting was carried out about 20 and 68 h after cutting. The same storage period was allocated to each sample in each rewetting.

2.2. Observations A plot of the thin layer drying pattern after rewetting 20 h after cutting is shown in Fig.

1. Other plots have been reported by Pitt and McGechan. 7 Three phases can roughly be identified in the drying pattern: an initial period of rapid drying as surface moisture evaporates; an intermediate period with a gradient steeper than that for unrewetted samples at a similar moisture content, presumably representing evaporation of absorbed moisture; finally, a return to the drying pattern for unrewetted samples, representing evaporation of tissue moisture. The critical point representing the changeover from drying surface moisture to drying absorbed or tissue moisture is curved and imprecise. However, inspection of a drying sample during this changeover period showed some patches with and some patches without droplets of surface moisture which would explain this imprecision. The rewetting carried out 1 h after cutting differed from the later rewettings in that the intermediate phase was almost non-existent, which is in agreement with the supposition that high moisture content grass does not absorb moisture.

2.3. Analysis Because of the imprecise changeover between each of the phases of the drying process,

it was difficult to estimate accurately the quantity of absorbed moisture. An attempt at

M. B. M c G E C H A N ; R. E. P I T T 71

2 4

2.2

2 .0 '7

v

1.6

0

'-' ~ "~ ' ,%_ ' ,FX

0 8

0.6 I I I I I I I I I I I I I I I I I I I O I I I

20 21 22 23 24 25 26 27 28 29 3 31 32 Time, h

Fig. 1. Absorption rate rewetting experiment with thin layer samples. Conditioned grass cut 17June 1987, rewet 20h after cutting.

, sample drying curve; , drying curve for unrewetted samples at moisture contents similar to the redried samples;

. . . . . . . . . , superimposed drying curve for zero storage period samples; +, zero storage period; E], 1-h storage period; A, 4-h storage period;

I, example of maximum vertical distances between two curves, representing quantity of absorbed moisture

such an estimate was made by superimposing the curve for zero length storage period on the curves for 1- and 4-h storage periods; the maximum vertical distance between the two curves was then assumed to be the quantity of absorbed moisture (Fig. 1). In fact samples with the zero length storage period had surface moisture lying on them for about 0.5 h (after return to the thin layer drying rig) and appeared to absorb about 0.05 kg(kgdm) -~ of moisture; 0-5 h of storage time and 0-05 kg(kgdm) -1 of absorbed moisture were therefore added to the values for the 1- and 4-h storage period samples also (Table 1).

In order to estimate the absorption time constant ("resistance") Ta, attempts were made to fit the following exponential relationship to the experimental data:

Ma . , -tiT = M . . . . . e ~ (1)

A relationship asymptotic to 5.0, which is the expected value of M . . . . . required a very large time constant with data available only for the beginning of the curve. As an alternative, an exponential relationship asymptotic to 1-0, i.e. roughly the quantity of water which was added to the samples, was considered more satisfactory (Table 1). Point estimates of the time constant with this asymptote (Table 1) have an overall mean of 15 h.

72 REWETTING OF GRASS SWATHS, 2

Table 1

Absorption rate experiments using thin layer drying equipment

Experiment cut date

Point estimates of time

constant T a Conditioned Rewetting

C, or time, Moisture Exponential Exponential hand cut, h after Absorption absorbed, asymptotic asymptotic

H cutting time, h kg(kgdm) -t to 5.0 to 1.0

17.6.87

25.6.87

C 20 1.5 0-16 46-1 8-6 0.15 49.2 9-2

4.5 0.24 91-5 16-4 0-20 110-2 20.2

H 1-5 0-14 52-8 10.2 0.10 74.2 14-2

4.5 0-13 170.8 32.3 0-13 170-8 32.3

C 44 1.5 0-21 35-0 6.4 0-19 38.7 7-1

4-5 0-24 91.5 16.4 0-21 104-9 19.1

H 1.5 0-11 67.4 12.9 0-12 61-7 11.7

4.5 0-17 130-1 24.2 0.18 122-7 22.7

C 20 1-5 0.11 67.4 12.9 0.14 52.8 9-9

4.5 0.28 78.1 13-7 0.24 91.5 16-4

H 1.5 0-09 82.6 15.9 0-08 93-0 18.0

4-5 0.19 116-2 21.4 0.23 95-6 17-2

C 68 1.5 0-12 61.7 11-7 0-16 46.1 8.6

4-5 0.31 70-3 12.1 0.33 65-9 11.2

H 1.5 0-15 49.2 9.2 0-12 61-7 11-7

4.5 0-30 72.7 12.6 0-30 72.7 12-6

Overall mean 15-0 Conditioned mean 12.4 Hand cut mean 17-4 1-h storage time mean 11-1 4-h storage time mean 18.8

17.6.87 experiment, rewet 20 h after cutting, mean 17.9 17.6.87 experiment, rewet 44 h after cutting, mean 15.1 25.6.87 experiment, rewet 20 h after cutting, mean 15-7 25.6.87 experiment, rewet 68 h after cutting, mean 11.2

M. B. M c G E C H A N ; R. E. P I T T 73

The mean time constant for conditioned samples (12.4 h) was markedly shorter than for hand cut samples (17-4 h), as might be expected. Absorption time constants were also slightly shorter in the later than in the earlier rewettings in both experiments. However, the point estimates of the time constants were much shorter for the 1-h than for the 4-h absorption periods. It is possible that absorption over the longer period was limited because of insufficient water, although some surface water was present when samples were returned to the thin layer drying rig as can be seen in the thin layer drying curves.

3. Investigation into runoff rate

3.1. Materials and methods

An exploratory investigation into the pattern of water runoff from grass was carried out in parallel with a field swath drying experiment on 30 June 1987; this exploited some of the equipment used in field swath drying experiments. 4"s

Two windrow swaths (approx. 0-8 m base width) were cut on 30 June with a steel spoke mower conditioner (cut width approx. 1.9 m) in a field of perennial ryegrass with a dm yield of 9.0 t ha -~. One swath was immediately hand raked into the form of a spread swath of width roughly equal to the cutting width of the mower. Three samples from each swath were transferred to baskets, weighed and the baskets replaced in the swath. The sample basket consisted of a light aluminium frame 1.5 m in length and 1.0m wide, supporting a light plastic net. Hence these baskets could contain the whole width of the windrow samples but only slightly more than half the width of the spread samples. On the day of cutting and each of the two subsequent days, one windrow sample and one spread sample were subjected in turn to the rewetting and runoff treatment. This consisted of removing the sample basket from the swath, weighing it, rewetting by pouring about 601 of water over it from a bucket, and returning it to the weighing balance. Weight readings were taken at intervals as the water ran off from the suspended basket. Frequent readings were taken at first, but the frequency was reduced as the quantity of runoff water declined. A final reading was taken about 2-5-3 h after rewetting and the sample returned to the swath. At the end of the field swath drying experiment, all six sample baskets were weighed. Hand grab samples were taken for oven moisture content determination after cutting and at the end of the experiment; after cutting, these were taken from the swath immediately outside each basket, and at the end after the final weighing from the grass in the basket.

3.2. Observations and analysis

The water runoff pattern for one of the six samples is shown in plots of dry basis moisture content (calculated from basket weights) against time in Fig. 2.

The first relationship fitted to the data consisted of two exponentials, one representing runoff and the other evaporation of surface moisture; the number of parameters was limited to four by making the curve asymptotic to the moisture content before rewetting M0:

M = 114o + Pie -e'J + P3 e-e'' (2)

The fitted parameters P~ and p~ in Eqn (2) should represent the maximum values M . . . . and M . . . . . . while 1/P2 and 1/P4 represent the time constants for runoff T3 and for evaporation of surface moisture (Table 2). The mean value of ~ , 1.58 kg(kg dm) -~, is similar to the value of about 1-5 kg(kg dm) -1 assumed in Part 1, and a large constant for

74 R E W E T T I N G OF GRASS SWATHS, 2

4.4

~... 4 2

~ 4 0

~ 3 8

.

U

~ 3 6

6 ~ 3 4 .

32 0

r ~ - - i r T ~ - - - r - - - ~ - T - -r- - 7 - - T V 7 • - ~ - - r - - ,

20 40 60 80 100 120 140 160 180 Time, min

Fig. 2. Runo f f experiment, 1 day after cutting, windrow swath. *, data points;

. . . . . . . . . , Eqn (2); . . . . , Eqn (3);

, Eqn (4)

evaporation is to be expected since the runoff experiments were deliberately carried out in a shady, wind-free hut to minimize evaporation. The mean value of P~, 0-58kg(kgdm) -~, is somewhat less than the expected maximum value for runoff moisture, Mr,max, of about 0-9 kg(kgdm) -~ assumed in Part 1. However, this is not surprising since some runoff took place after rewetting ceased and before the first reading was taken on the balance, while the sample basket was being mounted on the balance. The mean value of 1/P2, 3.44 min, is much less than 1.44 h previously supposed by van Elderen et al. 8 With the exception of the sample which had been wet by overnight rain,

Table 2

Runoff experiment, parameters fitted by a non-linear regression with a double exponential equation

Moisture content before

Days Time constants, rain rewetting, Initial grass after Swath M o, dry matter

cutting type P1 P2 P3 P4 Runoff, T 3 Evaporation kg(kg dm)- l weight, kg

0 Spread 0.530 0-316 1-74 0-00432 3.17 230 4.31 1.35 0 Windrow 0.682 0-304 1-44 0-00157 3.29 640 3.97 2.34 1 Spread 0-650 0-223 1-61 0.00184 4-49 540 3-32* 1.47 1 Windrow 0.494 0.281 1-46 0-00146 3.56 690 2.41 1.75 2 Spread 0-562 0.316 1-66 0.00226 3-17 440 2-11 1.24 2 Windrow 0.533 0.304 1-58 0-00170 3-29 590 2.54 2-07

Mean 0.575 0.291 1.58 0-00219 3-44 520 3.11 1-70

* Including some surface moisture from overnight rain

M. B. McGECHAN; R. E. PITT

Table 3 Runoff experiment, parameters fitted by a non-linear regression with three exponentials

75

Time constants, rain Days after Swath Eqn Fast Slow

cutting type no. PI P2 P.a P4 P5 P6 runoff runoff Evaporation

0 Spread 2 0.223 2.07 0-405 0-157 1-68 0-00380 0.48 6.4 260 0 Windrow 2 0-450 1.01 0.435 0.072 1.32 0.00088 0-99 13-8 1140 1 Spread 3 0.362 0.62 0-497 0.062 1-45 0-001130 1-61 16-1 1000 1 Windrow 2 0-308 0-71 0.350 0-067 1.33 0-00072 0-14 14.9 1390 2 Spread 2 0.229 1.60 0-466 0.151 1.58 0.00166 0.63 6.6 600 2 Windrow 2 0-300 1-47 0-418 0-111 1-49 0.00112 0.68 9.0 890

Mean 0-312 1-25 0.428 0.103 0-48 0.00153 0.76 11.1 880

time constants were slightly shorter for spread than for windrow swaths. This contrasts with the time constants for evaporation of surface moisture, which were markedly shorter for spread than for windrow swaths in every case.

Since the relationship given by Eqn (1) was not a very good fit to the data, and also because the time constant for runoff was found to be unexpectedly short, further relationships were fitted with three exponentials, as follows:

M = Mo + Pie -e~-' + P3e -P'' + ~ e -ea (3) n --O.IP~t n - - 0 - 0 0 1 t

M = Mo + P~e -p2' + R e " + R e (4)

Eqn (4) was chosen on the basis of the mean values of the coefficients in Eqn (3), because no convergence was obtained with six unknown parameters for the sample wet by overnight rain. In general, Eqns (3) and (4) gave much better fits to the data than Eqn (2), as would be expected (Fig. 2, Table 3). These results suggest that possibly two runoff processes take place after rewetting, which could be described as "fast runoff' ' and "slow runoff' ' . However, the mean time constant for slow runoff, 11.1 min, is still substantially faster than had previously been suggested by van Elderen.

4. Conc lus ions

A value of 15h (slightly shorter for conditioned grass and slightly longer for non-conditioned grass) is a reasonable first estimate of the absorption time constant Ta. This represents a slightly slower rate of absorption than had previously been assumed.

A mean value of 3.5 min for the runoff time constant T3 was estimated in one runoff experiment. Alternatively, by supposing that runoff consists of separate "fast runoff" and "slow runoff" processes, time constants of about 1 min and 11 min were estimated for each process. These values indicate that runoff after rainfall ceases is a very much faster process than has previously been assumed.

References

1 Pitt, R. E.; McGechan, M. B. The rewetting of partially dried grass swaths by rain: Part 1, Lumped and distributed models of moisture fluctuation. Journal of Agricultural Engineering Research 1990, 45:55-67

76 REWETT1NG OF GRASS SWATHS, 2

2 Lamond, W. J.; Spencer, H. B.; Glasbey, C. A.; Haughey, D. P. Field wilting and drying of grass in cool moist climate. Research and Development in Agriculture 1988, 5(1): 23-28

3 Lamond, W. Jo; Spencer, H. B.; Graham, R.; Moore, A. B. The effect of thin layer drying rate and swath architecture on the rate of swath drying under controlled conditions. Journal of Agricultural Science, Cambridge 1989, 113:59-65

4 Spencer, H. B.; Lamond, W. J.; Graham, R.; Glasbey, C. A.; Bowden, P. J.; Haughey, D. P. Silage wilting--a field trial comparing three different conditioners (1985). Departmental Note SIN/472, Scottish Institute of Agricultural Engineering, Penicuik, 1986 (unpublished)

s Spencer, H. B.; Lamond, W. J.; Graham, R.; Bowden, P. J.; Glasbey, C. A.; Haughey, D. P. Hay drying--a field trial comparing three different conditioners (1985). Departmental Note SIN/473, Scottish Institute of Agricultural Engineering, Penicuik, 1986 (unpublished)

• Hale, O. D. A laboratory technique for evaluating and comparing forage conditioning mechanisms. Journal of Agricultural Engineering Research 1984, 32:243-256

Pitt, R. E.; McGechan, M. B. The rewetting of partially dried grass swaths by rain. Departmental Note 1, Scottish Centre of Agricultural Engineering, Penicuik, 1987

8 van Elderen, E.; de Feijter, J.; van Hoven, S. P. J. H. Moisture in a grass swath. Journal of Agricultural Engineering Research, 1972, 17:209-218