Agricultural Water Management, 2 (1979) 225--232 225 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

TRICKLE IRRIGATION TIMING AND ITS EFFECT ON PLANT AND SOIL WATER STATUS

J. BEN-ASHER

Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus (Israel)

(Accepted 23 February 1979)

ABSTRACT

Ben-Asher, J., 1979. Trickle irrigation timing and its effect on plant and soil water status. Agric. Water Manage., 2:225--232.

Tomato plants on Sinai sand dunes were irrigated daily by drip irrigation. The irrigation was supplied during daytime hours for one field and a short t ime after sunset for the second. Results showed that daytime irrigation of soil with low water holding capacity increased the yield significantly and improved plant water potential as well as water use efficiency. The dominant component of water balance under these conditions was found to be deep percolation, which accounted for more than 70% of the water budget. Controlling this component rather than soil water status requires measurements of flux as input for man- aging the quantity of water to be applied. It is concluded that during hours of high net radiation flux, transpiration rate can best compete with deep percolation rate. Based on this conclusion, the use of net radiation flux as input is recommended for the best irriga- tion timing.

INTRODUCTION

Compared to conventional irrigation methods, a daily trickle irrigation cycle consists of a relatively long period of infiltration followed by a short period of extraction and redistribution. Under these conditions the water holding capacity of the soil becomes less important because water is supplied as required by the plants. Besides the lack of land for agriculture, the aware- ness of this fact and the development of irrigation systems capable of delivering water to the soil as much and as often as desired, are two of the main reasons for expanding the cultivated areas toward sandy soils. Trickle irrigation tech- niques have spurred the development of sand dune agriculture, but no field- scale investigation of the entire water balance under these new conditions has been reported. The objectives of this investigation were to estimate the various components of water balance under different, high frequency, irriga- tion regimes, and to propose some recommendations as to irrigation policy.

226

M A T E R I A L S AND METHODS

The effect of trickle irrigation timing was tested on a coastal area in the uniform coarse sandy soil of Nahal Sinai (latitude 31o04 ', longitude 33 ° 50~. This soil contains 97% sand and 1% clay and its bulk density is 1.63 g cm -3 on the average, with a volumetric moisture content of 6--8% at field capacity. The experiment was conducted with commercial Netafim I nozzles at a con- stant discharge rate of 2000 cm 3/h (+ 10%), the spacing was 0.5 m between emitters and 1.5 m between rows. The indicator plant was tomato (Solanum lycopersicum L. 'Arava S 5') which was planted on a grid of 1.5 × 0.25 m. Irrigation was given daily, starting at 6.00 a.m. in one t reatment and at 6.00 p.m. in the other. The basic assumption was that for soils of low water holding capacity dayt ime irrigation plants (DIP) enjoy a better soil water status than nighttime irrigation plants (NIP). The amount of water was 2000, 3000 and 4000 cm3/day per nozzle, in January, February and March, respectively. (This amount is equivalent to a one-dimensional irrigation of 2.7, 4.0 and 5.3 mm/ day.) Fig. 1 is a map of the gravimetric water sampling. In this figure the open circles represent the lower boundary of the sampling line and the solid lines are the volumes that are represented by a group of five sampling circles. The total volume is called a volume element of a single nozzle. The nozzles

Fig. 1. A seheme o f a tr iekle irr igated volume e lement , the sampling lines and the volume rep resen ted by each line. e , a tr ickle source; o, the lower l imit o f the sampling line; r = the d is tance f r o m the tr ickle souree (era); Z = the d e p t h (era).

1 The c o m p a n y name is given for the benef i t o f the reader and does no t infer p re fe rence fo r this vendor .

227

were spaced symmetrically such that in order to understand the behavior of the entire field it was sufficient to analyze the water balance in a single element. Four samples a day were taken with a Veihmeyer tube throughout five measuring days. The sampling times were: (1) before irrigation, at the end of the irrigation cycle; (2) after irrigation, at the beginning of redistribution; (3) sunrise and sunset {these measurements enabled separation of the water balance components into its nightly and daily parts, and estimation of the amount of water lost with and without evapotranspiration). The fourth measurement was taken at noon as a control of the changes. Specific tests for standard deviation in the measuring method showed that after irrigation the standard deviation was 24%, and before irrigation, at the end of the cycle, it was 17% of the total water volume found in a volume element. Plant response to its integrated environmental conditions was specified by yield and plant water potential taken with a "Pressure-bomb". Evapotranspiration was measured by the Bowen ratio energy balance method from the entire field and then calculated per volume element (Ben-Asher et al., 1978). Class A pan evaporation was measured every day and then used for normalization of actual evapotranspiration at different dates and atmospheric conditions.

R E S U L T S A N D D I S C U S S I O N

Soil water c o n t e n t

From Table I it seems that at the beginning of evapotranspiration, an

average of about 11 and 9 1 of water were available for DIP and NIP, respecti- vely. The reason for this difference is shown in the last row of Table I.

T A B L E I

Water volume at the beginning and the end of evapotranspiration (cm ~)

Date 6--7 Jan. 15--16 F e b . 22--23 Feb. 17--18 March 22--23 March Irrigation timing Day Day Night Day Night Vo]. of water

application 2000 3000 3000 4000 4000

End of infiltration 10163.0 10943.9 10685.1 11750.0 10413.1 Moisture content 10.3 11.1 10.9 12.0 10.6

Beginning of evapo- transpiration 10163 10943.9 9577.1 11750.0 8351.8

Moisture content 10.3 11.1 9.7 12.0 8.5

End of evapo- transpiration 7679.8 9029.8 7784.9 11180.6 6751.3

Moisture content 7.8 9.2 7.9 11.4 6.9

Losses between irrigation and beginning of evapotranspiration* 0 0 1108.0 0 2061.3

(36.9) (51.5)

*Percentage of water losses from total applied water is given in parentheses.

228

When irrigation was applied at night, about 35b-50% of the water was lost by deep drainage below the root zone between water application at 6.00 p.m. and the beginning of evapotranspiration at 6.00 a.m.

At the end of the simultaneous processes of infiltration, redistribution and evapotranspiration, about 9 and 7 1 of water were found in the respective root zone of DIP and NIP. Note that for DIP this is not the end of the irriga- t ion cycle and there are a further 12 h of percolation and redistribution be- fore the application of the next irrigation. Throughout these 12 night-hours the rest of the water was drained below the bot tom of the root zone such that the complete water loss was about the same for DIP and NIP. However, since the sequence of the processes was different for NIP (starting transpiration after percolation) and DIP (starting transpiration before percolation), the latter enjoys a better soft water status than NIP even though the amount of water used throughout the complete irrigation cycle was about the same [8250 1 (+ 17%)] for both treatments. In addition, it was not possible to detect significant differences in the soil water content after application of 2000 and 4000 cm 3 to a volume element of DIP or NIP. The average water content of a single volume element was 10.8 + 2.6%, which is within the standard deviation of the measuring method. Thus, in agreement with Raw- lins (1973) we can say that since sufficient water was applied to meet evapo- transpiration demands (Ben-Asher et al., 1978) the addition of extra water to a volume element did not significantly increase the soil water content , and it simply allows more water to escape from the bot tom of the root zone.

These losses are typical for softs of coarse texture and low water holding capacity. The water flow rate within the saturated zone around the trickle source is close to its saturated hydraulic conductivity and is, therefore, acting like an open vertical pipe that transfers extra water from the surface to some deeper layers.

Plant water potential

In Table I, the amount of water in a volume element is also given in its equivalent values of moisture content . There, throughout the daytime hours, a decrease in average moisture content from 11.1 to 9.5% and from 9.1 to 7.4% was found for DIP and NIP, respectively. According to Fig. 2 these changes in soil water content are associated with a respective increase in plant negative water potential. The measured negative potential of DIP increased from 8 to 10 atm and that of NIP increased from 10 to 13 atm at a constant evapotranspiration rate of 0.15 mm/h around noontime. Thus, for constant atmospheric conditions NIP consumed more energy for water uptake than DIP. From the daily course of plant water potential (Fig. 3) it seems that this fact was more pronounced around noontime when the evaporative ability of the atmosphere was stronger. Throughout four measuring days at noon- time, NIP's water potential was higher than DIP's, while early in the morning and late in the af ternoon it was not possible to detect differences between

229

c

13_

8

z 6

I f I I

Z~

I I I I 0.06 0.08 O.lO 0.12 Moislure Content (cmSx cm -3)

Fig. 2. The p lan t ' s water po ten t i a l as a f unc t i on of mois tu re c o n t e n t at a co n s t an t evapo- t ranspi ra t ion rate o f 0.15 m m h. (o NIP; 23.3.71, A DIP; 16.2.71, o DIP; 18.3.71)

-6 ¢-

#_

(D

Z

141 I I I I 1 !

P 12

I0

6

21 15-16 2 1971

0 I I I I I I 0 6 I0

I I I I I 1 I I I I I I I I I

/ 17- 18.3i 1971

I I I I I I 0 6 I0 14

I I I I I

I I 18 O6

2 2-2~,211971 2 2-23.3.1971

I I I I I I I I I I I I Iq 18 0 6 I0 14 18 I0 14 18

Time of Day Fig. 3. The plant's water potential as a function of daytime hours for DIP (a) and NIP (A).

t he two irr igat ion regimes. Thus , as a resul t o f be t t e r soil mois tu re condi t ions , 40 tons o f t o m a t o e s were harves ted per ha o f DIP and a b o u t 30 tons f ro m NIP. These d i f fe rences were signif icant at a level o f 5--10%.

Water budget o f an irrigated volume element

The wa te r budge t is given b y the hydro log ic wa te r balance. This equa t ion expresses the law o f c o n t i n u i t y as appl ied to a vo lume e lement , and is given by

Et* + A S n + Z + I = 0 (1}

230

where I is water application from a single nozzle, Et* is evapotranspiration from the net evaporative area, ASn is the change in soil water content, and Z is deep percolation (all units are in cm 3 ).

In Eqn. (1), Et was measured directly by the Bowen ratio energy balance method. The results of these measurements have been published elsewhere (Ben-Asher et al., 1978). In this study, M, the ratio of dry area to total area, was found to be 0.66. It was also concluded that this portion of the field did not evaporate. Thus, the only source of water for evapotranspiration was the wet portion which is called the net evaporative area and occupied (l--M) of the total field. Furthermore, because of the symmetric spacing between the nozzles it was possible to calculate the contribution of each nozzle and its wet portion to the total measured evapotranspiration, according to Eqn. (2)

Et* = Et-nr2/(1--M) (2)

where Et* is the evapotranspiration from a single volume element (cm3), Et is the measured evapotranspiration (mm), r is the radius of the wet portion around the nozzle (25 cm in this study), and (l--M) is the ratio of the wet area to total area (1/3 in this study).

ASn in Eqn. (1) was measured gravimetrically at the end of each irrigation cycle, the amount of water applied,/ , was defined according to the duration of irrigation, and Z was eliminated from Eqn. (1) as the only remainder to have the total water budget, as given in Table II.

TABLE II

Daily water budget of a volume element (cm 3)

Treatment I Et* ASn Z

DIP 2000 (100%) 394 (20%) 87 ( 4 % ) 1519 (76%) DIP 3000 (100%) 720 (24%) 306 (10%) 1974 (66%) DIP 4000 (100%) 459 (11%) 500 (13%) 3041 (76%) NIP 3000 (100%) 400 (13%) 99 ( 4 % ) 2501 (83%) NIP 4000 (100%) 687 (17%) 3 3 8 ( 9 % ) 2975 (74%)

Table II shows that within 24 h more than 90% of the applied water was lost. In the irrigation cycle the dominant process for both treatments was deep percolation. Table II also shows that changing the water application by 1000 cm 3 (from 3000 to 4000 cm 3) increased the deep percolation by about 500 and 1000 cm 3 for NIP and DIP, respectively, while the water content in the volume did not change significantly. This fact again emphasizes the point raised by Rawlins (1973} that adding extra water to a volume element under high frequency irrigation allows more water to escape from the root zone.

It should be mentioned that the evapotranspiration derived from the energy balance measurements were made at different dates. Thus, to compare the two treatments with respect to evapotranspiration it was necessary to use a normalizing procedure which would account for the atmospheric conditions

231

at the time of measurements. Class A pan evaporation was used for this pur- pose.

The effect of irrigation timing on Et can be seen from Table III. The average ratio of Et to Ea was about 1.0 for DIP and less than 0.7 for NIP. This is another way of saying that evapotranspiration flux for DIP was mainly limited by the evaporative ability of the atmosphere, while that of NIP was also limited by soil moisture content.

T A B L E III

The ef fec t of irr igation t iming on normal ized evapotranspira t ion (Et /Ea)

T rea tmen t

DIP NIP

2000 3000 4000 3000 4000

Et (ram) 2.0 3.5 2.3 2.0 3.4 Ea (ram) 2.0 3.8 2.2 3.9 4.0 E t /Ea 1.00 0.92 1.06 0.52 0.86

Irrigation water use ef f iciency

The criterion for water use efficiency (/eft) is defined by Eqn. (3)

/eft = Et*/(Ea* + Z) (3)

which is the ratio of water losses through evapotranspiration process (Et*) to the sum of deep percolation (Z) and class A pan evaporation (Ea*). This criterion can take into account the limiting factor for evapotranspiration and therefore the irrigation regime. For DIP with Et* ~Ea*, Eqn. (2) becomes that of Hillel (1971) and for NIP with Et* < Ea* the soil water status would be reflected in the low ratio Et/Ea and expressed as low water use efficiency in Eqn. (3). Table IV summarizes the experimental values of irrigation ef- ficiency.

T A B L E IV

The e f fec t o f irr igation t iming on water use eff ic iency (Ieff) and its componen t s

Month Jan. Feb. March Water appl icat ion 2000 3000 4000 Trea tmen t DIP DIP NIP DIP NIP Et* (cm 3 ) 394 720 400 459 687 Ea* + Z (cm 3) 1912 2756 3270 3474 3773 Ief f (dimensionless) 0.21 0.26 0.12 0.13 0.18 Ea* (cm 3 ) 393 782 769 433 799

Ea* in units o f vo lume is the p roduc t of daily Ea uni t length and net evaporat ive area as def ined in Eqn. (2).

232

In this table the best comparable treatments are those of February, having similar water application and class A pan evaporation. According to the sug- gested criterion the water use effciency of DIP was 14% larger than that for NIP in February, and 5% larger on the average. DIP's low efficiency in March is explained by a low correlation between the amount of water applied and the atmospheric conditions of the measuring day. This low correlation can be improved by updating records of the atmospheric conditions and the respect- ive water application on an hourly basis as required by high frequency irri- gation.

CONCLUSIONS

Three aspects of daily drip irrigation were found to be affected by its timing. These are: yield, plant water potential, and irrigation efficiency. Generally, a bet ter water use was achieved by decreasing water f low below the root zone; this happened when water application and extraction occur- red simultaneously. The conclusion to be drawn here is that further improve- ment of water use efficiency can be gained by delivering water on the basis of soil water flux rather than soil water content or soil water potential. This policy was first recommended by Rawlins {1973) in his analysis of high fre- quency irrigation and was experimentally found to be valid in this study. Obviously, to prevent deep percolation, the best time to irrigate is when roo t uptake is at its maximum. This is when the evaporative ability of the atmosphere is at its peak. A good and applicable criterion for this could be the net radiation flux. Since there is no reason to irrigate in excess of the evapotranspiration (except for salt leaching), the net radiation flux, with some empirical calibrations, can help to decide when to irrigate by using the water flux term. This is instead of the soil water content or potential which were used for conventional irrigation intervals with a water extraction- dominant process. This approach would result in water uptake at a relatively high tension early in the morning when uptake rate is low. The second stage in the irrigation cycle would be water application during hours of high solar radiation flux and water consumption. At this time the evapotranspiration rate can best compete with the deep percolation rate and transpiration occurs under relatively low soil water tension.

REFERENCES

Ben-Asher, J., Fuchs, M. and Goldberg, D., 1978. Radiation and energy balance of sprinkler and trickle irrigated fields. Agron. J., 70: 415--417.

Hillel, D., 1971. Soll and Water, Physical Principles and Processes. Academic Press, New York, N.Y. and London, 288 pp.

Rawlins, S.L., 1973. Principles of managing high frequency irrigation. Soil Sci. Soc. Am. Proc., 37: 626--629.