Soil and plant water status under sprinkling and trickling

Agricultural Water Management, 1 (1976) 33--40 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

33

SOIL AND PLANT WATER STATUS UNDER SPRINKLING AND TRICKLING

S.D. GOLDBERG, J. BEN-ASHER and B. GORNAT

Department of Irrigation, Hebrew University of Jerusalem, Jerusalem (Israel) (Received 15 December 1975)

ABSTRACT Goldberg, S.D., Ben-Asher, J. and Gornat, B., 1976. Soil and plant water status under sprinkling and trickling.Agric. Water Manage., 1 : 33--40.

In a comparison of methods of irrigating tomatoes on the sand dunes of northern Sinai (EI-Arish region), yields obtained by trickling were higher than those by sprinkling. The present study attempts to explain these results from a physical point of view. Before each irrigation and during a complete irrigation cycle measurements were made of soil moisture content, moisture tension in the root zone, and plant water potential. The amount of water applied was based on Class A pan evaporation. At 24 h after the end of an irrigation the soil moisture content was 4% by weight, regardless of the quantity of water applied. The soil moisture tension and the plant water potential were similar for both methods during the first 24 h after irrigation, but the values rose gradually and were higher at the end of the sprinkle irrigation cycle which lasted 3 days, than at the end of

the daily trickle irrigation cycle. These differences in soil moisture tension affected the plant water potential and in turn plant development and yield.

INTRODUCTION

In r e c e n t years, trickle irrigation has b e c o m e widely i n t r o d u c e d as an i m p o r t a n t m e t h o d o f w a t e r a p p l i c a t i o n . It has b e e n p a r t i c u l a r l y successful in regions w i t h s a n d y soil and saline irrigation w a t e r ( G o l d b e r g et al., 1970). One o f the e x p l a n a t i o n s o f f e r e d f o r the beneficial e f f e c t o f trickling has b e e n t h e p r e v e n t i o n o f leaf s c o r c h t h r o u g h e l i m i n a t i o n o f leaf wetting. This e f f e c t is especially p r o n o u n c e d w h e n irrigation w a t e r is saline, b u t its i m p o r t a n c e in t h e case o f non-saline w a t e r has n o t y e t b e e n c o n c l u s i v e l y e s t a b l i s h e d ( G o r n a t et al., 1973). A n o t h e r a d v a n t a g e o f trickling is t h a t t h e soil m o i s t u r e t e n s i o n achieved b y daily irrigations is l o w e r t h a n t h a t o b t a i n e d b y o t h e r irrigation m e t h o d s in w h i c h t h e s a m e q u a n t i t y o f w a t e r is applied, b u t at less f r e q u e n t intervals ( G o l d b e r g a n d Shmueli, 1970). Trickling a n d sprinkling can be c o m p a r e d in t w o ways. T h e first regards sprinkling as a m o v a b l e irrigation s y s t e m in w h i c h t h e field is p e r i o d i c a l l y irrigated, w h e r e a s trickling is r e g a r d e d as a f i x e d s y s t e m in w h i c h t h e field is s u p p l i e d dally w i t h t h e w a t e r r e q u i r e m e n t o f t h e c r o p . A c c o r d i n g to this

34 approach, the comparison should consider the methods as they are normally applied in the field. The second method is based on water applications using identical irrigation intervals for both methods, thus providing more similar conditions for comparison. The disadvantage of the former approach is that irrigation frequency is introduced as one of the factors affecting the results. The objective of the present study was to determine the effect of irrigation method and frequency of application, as an integral part of the method, on the status of water in the soil and plant, in order to provide an explanation (in addition to that provided by Gornat et al., 1973) for the differences in development and yield of t o m a t o plants. METHODS The experiment was conducted on tomatoes planted on the coastal sand dunes near E1-Arish in northern Sinai. Two irrigation treatments were tested: daffy trickling with emitters with a discharge rate of 2 1 per h, and sprinkling at 3-day intervals with sprinklers adapted for windy conditions with an application rate of 7 mm per h. Daily sprinkling was not given since this is known to cause leaf scorch and wilting. The quantity of water applied was 1.4 times the evaporation from a Class A pan located in the experimental field. The total seasonal water application to the crop was 1600 mm. During the period of measurement, December--January, the daily application was 3--4 mm. This a m o u n t is equivalent to a discharge of 2--3 1 per emitter, so that in the trickier m e t h o d the irrigation lasted 60--90 min each day, and with sprinkling, 90--120 min once every 3 days. The soil is sandy in texture, having a cation exchange capacity of 2--5 meq./100 g soil, and a bulk density of 1.63 -+ 0.04 g/cm 3. The water content at field capacity is 4--5% (on a dry weight basis) and the electrical conductivity of a 1:1 soil--water extract is 0.5--1.0 mmhos/cm. The irrigation water has a C1 content of 820 ppm and an E.C. of 3.6 mmhos/cm. Soil moisture was determined gravimetrically to a depth of 50 cm every 10 cm, at distances of 0, 15 and 25 cm from the emitters. The sampling points for both irrigation methods were selected in the immediate vicinity of a tomato plant stem. The soil moisture conditions produced by the two irrigation methods were compared by calculating the a m o u n t of water in the root zone. The root zone was found to be limited to a depth of 30 cm and to a radius of 25 cm from the plant stem or emitter. The a m o u n t of soil moisture, 0, under trickling was calculated by numerical solution of the integral: V = l l f R o fo z rO d z d r where V = volume of water in the root zone (cm 3) R = horizontal distance from the emitter (cm) Z = sampling depth (cm) 0 = water content (cm 3/cm 3)

(1)

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In solving the integral, the following parameters were used: R = 25 cm, Z = 30 cm, Ar = 4 cm, and Az = 10 cm. The moisture content, 0, was obtained by interpolation from the measured moisture contents. The sprinkling intensity per unit area was obtained from the sprinkler manufacturer's specifications. The corresponding values for trickling were calculated by assuming that the average radius of the saturated area, through which infiltration takes place, is 4 cm (actually, it increases gradually from 0 to 6 cm, depending on the duration of water application). Accordingly, the sprinkle and trickle irrigation intensity was 1.2 X 10 -2 cm per min and 4.0 × l f f 1 cm per min, respectively. The soil moisture regime in the root zone was measured by means of tensiometers inserted to a depth of 20--25 cm near the emitter. In the sprinkleirrigated plots, the tensiometers were inserted in the soil next to the stem of a t o m a t o plant, assuming a uniform distribution of water over the irrigated area. The pre-irrigation values of soil moisture tension were used to characterize the average tension during the irrigation cycle. Changes in soil moisture tension from the start of one irrigation to the start of the next were measured at short intervals. Plant response was evaluated by calculating the relative water c o n t e n t (RWC) of the leaves. Leaf samples were detached from the plant, weighed on an analytical balance, and then placed in water for a few hours. The saturated leaves were weighed, oven-dried at 105°C and weighed again. The RWC was calculated from the following equation: RWC = (Wf--Wd)/(Ws--Wd)

(2)

where Wf = fresh weight Ws = saturated weight Wd = oven-dry weight Plant water potential was measured on 2 days by means of a pressure bomb developed by Scholander, et al. (1965). On the first day of measurement, both treatments were irrigated. The second day of measurement was after a 3-day period during which only the trickle-plots received water. In order to obtain the diurnal changes in water potential, the plants were sampled from each treatment several times during the daylight hours. The measurements were made on y o u n g branches from two plants, and each reading of water potential was repeated twice. RESULTS AND DISCUSSION

(a) Soil water status

The change in soil moisture tension during a full irrigation cycle is shown in Fig.1 which illustrates the tension changes occurring in a sprinkling cycle which spans three trickling cycles. In both methods, the water application was calculated on the basis of 3.5 m m / d a y . During the first day after irrigation,

36

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Fig. 1. Soil moisture tension (in atm. ) during one sprinkle irrigation cycle and three trickle irrigation cycles.

the differences in moisture tension between the two methods did n o t exceed the estimated error of + 0.02 atm. On the second day, the tension increased in the sprinkled plot, while in the trickled plot the process observed on the previous day of irrigation repeated itself. The most rapid increase in soil moisture tension occurs on the third day of the sprinkling cycle, during which the greatest differences in tension were recorded between the two irrigation methods. The results of the first day of the irrigation cycle indicate a basis for comparing the two methods according to the approach which maintains that they must be compared under identical conditions, including the frequency of water application, so that the only variable will be the irrigation method itself. The results of the complete irrigation cycle provide a basis for comparing both methods in line with c o m m o n field practice in which irrigation frequency (which is relatively low in sprinkling compared to trickling) is an integral feature of the system. Since, according to Fig.l, there were no significant differences in soil moisture tension during the first day after irrigation by both methods, whereas on the third day of the sprinkling cycle very large differences were recorded, it seems that the differences are the result of irrigation frequency and are not

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necessarily due to the irrigation method in the strict sense of the word. In the case of the soil moisture content near the plant stem or emitter (Fig.2), the differences between the two methods were more pronounced. The lower soil moisture content values under sprinkling may be attributed to the fact that the rate of water application is lower than the final infiltration capacity. Under these conditions, the maximum moisture content in the soil will be that at which the unsaturated hydraulic conductivity is equal to the sprinkling intensity. Fig.3 shows that the moisture content (by weight) will be lower than or equal to about 9% under sprinkling. In the case of trickling, this level is reached at a depth of 20 cm below the emitter (after 1 h of irrigation). The moisture content in the wetted profile decreased gradually from saturation directly below the emitter, to 9% at a depth of 20 cm. The above results can be explained by reference to the soil moisture retention curve (Fig.3) which shows that with a change in moisture content between 5 to 12% by weight, the moisture tension is constant within + 0.02 atm. The moisture contents measured beside the tensiometers in both irrigation methods were within the above-mentioned range.

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38 The pre-irrigation soil moisture content measured near the plant stem at the end of the irrigation cycle is given in Fig.4. In the trickle-irrigated plots, the soil moisture was sampled near the emitter. At this location, water spreads in a three-dimensional pattern in the soil profile, and therefore a higher moisture content may be expected at this point than at any distance from the emitter. In sprinkling, a two-dimensional distribution of water is assumed, and the expected water profile is uniform for all sampling points within the irrigated area. In order to obtain a more complete picture of the water status in the root zone as defined in Methods, the amounts of water corresponding to the volumetric water contents were calculated for trickling by using eq. (1), and for sprinkling by two-dimentional integration. The contents were 4500 + 700 cm 3 under trickling and 3400 -+ 500 cm 3 under sprinkling. (b) Plant water status

The course of plant water potential was measured after an irrigation applied at night (Fig.5). The values were similar for both irrigation methods, and there was little change in potential during the early morning hours. The absolute values were relatively low since the measurements were made on a cloudy day. Since plants do not transpire at night, the energy required to maintain the plant water potential is practically equal to the suction force of the soil. Thus, the values of water potential observed during the night represent the total soil moisture tension. According to this assumption, the total soil water potential one day after sprinkling was equal to that measured under trickling, whereas on the third day the potential was more negative than that measured under trickling (Fig.6). The potential at the beginning of the third day after sprinkling was similar for both irrigation methods (Fig.6). Later in the day, the trickle plots were irrigated, resulting in the development of differences in plant water potential. Whereas the potential in the sprinkle-irrigated plants continued to rise rapidly, the rate of increase in potential was more moderate in the trickle-irrigated plants. These results were in agreement with the RWC of the t o m a t o leaves at the end of the irrigation cycle. In the case of plants irrigated by trickling it was 0.994. This indicates that the rate of water supply from the soil to the leaves was very close to the rate of water supply from the leaves to the atmosphere, or in other words, that the rate of water loss approached potential transpiration. The corresponding RWC value for sprinkled plants was only 0.853. It is generally considered that at a RWC of about 0.8, plants will show severe signs of water stress. The relation between plant water status and rate of growth of different plant parts has been extensively studied, and much information is available today. Proper water supply stimulates cell division and enlarges each individual

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cell before it divides or matures by stretching the cell walls as the turgor pressure increases. On the other hand, water stress reduces the rate of cell division and growth of the individual cell as the turgot pressure drops. In the light of this, it seems that the criteria selected to characterize the plant water status, that is, total water potential and RWC which is frequently called relative turgidity (RT), may serve to explain why the height of the trickled t o m a t o plants was 114 cm while the sprinkled plants were only 76 cm high. (The least significant difference at the 5% level of probability was 12.5 cm). It can also explain the difference in yields: 79 tons/ha with trickling and 30 tons/ha with sprinkling (least significant difference = 1 ton/ha).

CONCLUSIONS

In this study an a t t e m p t has been made to explain, from a physical point of view, the differences in yields between plants irrigated by trickling and sprinkling with saline water. Some of the rsults have been explained in another paper (Gornat et al., 1973) by the presence of salts in the water and the effect of wetting of the foliage by the saline water. Another explanation, and perhaps no less important, has been found in the data reported in the present paper. These indicate that in sandy soils in which the water movement below the root zone is rapid, there is a sharp increase in soil moisture tension 2 days after irrigation, even if the evapotranspiration rates are low and do n o t exceed 2--3 m m / d a y . The increase in soil water tension expresses itself in a marked

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reduction in plant water potential, in the rate of plant growth, and in the yields produced. However, the continuous measurements of water status in the soil and plant have shown that it is possible to reach the same high potentials in the soil and plant by sprinkling as by trickling, if a large amount of water similar to that used in this study is also applied daily by sprinkling. REFERENCES Goldberg, D. and Shmueli, M., 1970. Drip irrigation-- A method used under arid and desert conditions of high water and soil salinity. Trans.Am. Soc. Agric. Eng., 13: 38--41. Goldberg, D., Gornat, B., Shmueli, M., Ben-Asher, J. and Rinot, M., 1970. Increasing the agricultural use of saline water by means of trickle irrigation. Proc. 6th Amer. Water Resources Conf., Las Vegas, Nevada. Water Resour. Bull., 7(4) August 1971, pp. 802--809 Gornat, B., Goldberg, D., Rimon, D. and Ben-Asher, J., 1973. The physiological effect of water quality and method of application on tomato, cucumber and pepper. Proc. Am. Soc. Hort. Sci., 98: 202--205. Scholander, R.F., Hammel, H.T., Bradstreet, Edda D. and Hemmingsen, E.A., 1965. Sap pressure in vascular plants. Negative hydrostatic pressure can be measured in plants. Science, 148: 339--346.