J. Agronomy & Crop Science 162, 135—140 (1989)© 1989 Paul Parey Scientific Publishers, Berlin and HamburgISSN 0931-2250

Haryana Agricultural University, Hisar, India

Development and Testing of Root Effectiveness Functionfor Soil Water Uptake

R. K. MALIK, N . K . NARDA and V. V. N. MURTY

Authors' addresses; Dr. R. K. MALIK, Agricultural Engineer, Dryland Agricultural Research Project,Haryana Agricultural University, Hisar (India), Dr. N. K. NARDA, Professor of Soil and Water Engineering,Puni'ab Agricultural University, Ludhiana (India) and Dr. V. V. N. MURTV, Professor of Soil and WaterEngineering, Punjab Agricultural University, Ludhiana (India).

With one figure and 4 tables

Received July 27, 1988; accepted October 27, 1988

Abstract

For soil water uptake studies under cropped conditions, root effectiveness for soil water absorption is of greatsignificance. For deveiopment of root effectiveness function, information on the age of roots present at anysoil depth at any time during the crop growth period is needed. For estimation of root age and rooteffectiveness function, a methodology has been developed. Wheat crop was used as a test crop. It has beenobserved that root effectiveness for water uptake remains constant during the early period of root growth andthereafter it decreases exponentially with root age. The application of the proposed root effectiveness functionwas tested successfully using macroscopic model of soil water dynamics which yielded 7.83 per cent variationbetween observed and simulated soil water content on overall basis in the crop root zone for the entire cropgrowth season.

I. Introduction

Soil water depletion under cropped conditionsis controlled by soil, plant and climatic factors.The macroscopic scale soil water dynamicmodels which simulate soil water depletionunder cropped conditions using different rootsink terms have been reviewed by MOLZ (1981)and MALIK et al. (1986). Most of these modelsneglect the soil water absorbing characteristicsof the plant roots.

When the plant roots are young and fieldwater conditions are optimal, the plant rootscan be considered as fully effective for wateruptake but plant root and soil water conditionschange during the crop growth period. Underthese conditions, the soil water uptake by plantroots needs to be modified by incorporatingthe factors which account for the decreasingwater absorption by the roots because of root

ageing during the crop growth period and di-minishing soil water during each drying cycle.Proposals for soil water availability to plantroots in relation to available soil water de-veloped by several investigators have been welldocumented by MALIK et al. (1988). But scarceinformation is available on root effectivenessfor soil water uptake. KLEPPER and TAYLOR

(1979) observed that root effectiveness for soilwater absorption decreases appreciably withroot age. NARDA and CURRY (1981) reportedthat root effectiveness for soil water absorptiondecreases approximately to one-thirteenthtimes during soybean crop growth period.NEUMANN (1982) reported that soil water ab-sorption in the older regions of root growth isslower than in the young and rapidly extendingregions. SHARMA and CHAUDHARY (1983) ob-served that during later wheat growth period

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136 MALIK, NARDA and MURTY

T.ible 1. Determinants

Soil depth(cm)

153045607590

105120135

of root age at different

a

7.64782.44670.6044

-2.8134-2.4501-5.3425

-15.2369-8.0856

-21.1442

soil depths

Values of coefficientsb

-0.0262-0.0579-0.1752-0.1490-0.1855-0.2185-0.1480-0.3155-0.1385

c

3.94812 X 10"-'3.98175 X 10"^6.62533 X 10"^4.42746 X 10"^4.36124 X 10"'4.56580 X lO"-'4.40610 X 10"'4.71770 X 10"^3.71337 X 10-5

despite higher rooting density in the upper soilsegment, soil water uptake rate was consider-ably lower because of the older roots.

MoLZ (1981) suggested that macroscopic soilwater dynamic models could be improvedfurther if appropriate considerations are givento the activity of plant roots. D E JONG andHAYHOE (1984) used different root activity val-ues for soil water uptake that remain constantover a certain interval of time. JUNG and TAY^

LOR (1984) used plant age instead of root agefor soil water uptake studies, and further sug-gested that age of the roots must be accountedfor the better understanding of dynamicchanges in soil water under cropped condi-tions.

An attempt has been made in this investiga-tion to develop a methodology for estimationof root age and the root effectiveness functionfor soil water absorption by plant roots.

II. Methods and Materials

An experiment was conducted in sandy loam soil atPunjab Agricultural University, Ludhiana (India),for two winter seasons. The data of one year wasused for development of the root effectiveness func-tion while of next year was used for testing it. Thewheat crop was grown using recommended packageof management practices. The moisture from eachsoil layer of 15 cm upto 150 cm depth was gravimet-rically determined periodically. Root samples col-lected periodically from each soil layer of 15 cmwere washed, cleaned and root lengths were esti-mated by modified NEWMAN (1966) proposed byMARSH (1971) on 1 x l cm grid size. The root lengthof each sample were transformed into root lengthdensity i.e. root length per unit volume of soil.

III. Results and Discussion

Development of methodology for estimationof root age:For developing root effectiveness function, in-formation on the age of roots present at anysoil depth at any time during crop growthperiod is essential. Age of the individual rootspresent at any soil depth is difficult to ascertainsince new roots are being added continuouslyduring any time interval through root prolifer-ation. No information is available in the pub-lished literature about the root age estimation.A methodology based on weighted averagewas developed and used for development ofroot effectiveness function of soil water ab-sorption.

For determination of root age at each soildepth, the field observed root length densitieswere linearly incremented daily between anytwo consecutive root samplings during cropgrowth period. Each increment in root lengthdensity was multiplied by the correspondingroot age of the incremented root length densi-ty. The corresponding age was determined forany root length increment by subtracting theday of its generation from the day on whichroot age is to be determined. The products ofeach incremented root length density and thecorresponding age were summed up. Thesesummations were divided by the root lengthdensity of which root age is to be estimated fordifferent soil depths. These root ages at differ-ent soil depths during the crop growth periodwere utilized for developing empirical relationof the foiiowing form.

A, = a -h bt + ct^ (1)

Development and Testing of Root Effectivencsi, Function for Soil Water Uptake 137

Table 2. Specific water uptake rate, root age and f(r) at 88th day of crop growth

Soil depth(cm)

Specific water uptake rate(cmVcmVcm root/day)

Root age(days)

f(r)

3045607590

105

8.0311 X10.7380 X13.2790 X14.4504 X22.0145 X26.2345 X

2821

181511

6

0.30610.40930.50620.55080.83911.0000

Where Ar is the root age (days) at different soildepths, t is the time (days) after crop sowingand a, b and c are constants which are listed inTable 1.

From eq. (1) root age at any soil depth at anytime during the crop growth period can beestimated. The non-existance of roots at anysoil depth is expressed by negative value of Aj..Development of root effectiveness functionf{r):The function f(r) was estimated from thedepthwise soil water extraction patterns duringthe wetter part of the drying cycle when cropcanopy covers intensively the entire field sur-face and the root system is fully developed.The soil moisture samplings were carried outwhen gravitational water has drained out. Thewater extraction from the uppermost soil layerhas not been taken into consideration for esti-mation of f(r) as the uppermost soil layer maybe subjected to evaporation even if the cropcover IS fully established. For development off(r) it has been assumed that moisture changesm the soil due to uptake by the roots are muchlarger than those associated with the matricand gravitational gradients. Depthwise soil wa-

ter extractions and root length densities wereused to estimate the corresponding specific soilwater uptake rates. Corresponding root ages atdifferent soil depths were estimated from e.q.(1) for the day for which specific soil wateruptake rates were determined. Since no defi-nite information is available at what stage ofroot growth, roots begin to limit soil waterabsorption, it has been assumed that the rootspresent in the bottom most extending regionare fully effective for soil water absorption.The depthwise specific soil water uptake rates,corresponding root ages and the root effective-ness at 88th and 105th days of crop growthperiod are given in Tables 2 and 3, respec-tively.

From the values of root age and root effec-tiveness reported in Tables 2 and 3, followingrelationship (Fig. 1) was established.

f(r)= 1A2255 €-'''"''^ (2)

Where f(r) is the root effectiveness function(fraction). The Figure 1 shows that root effec-tiveness decreases exponentially after initialstages of wheat root growth. The function f(r)is applicable at any nodal point of the root

Table 3. Specific water uptake rate, root age and f(r) at 105th day of crop growth

Soil depth(cm)

30

456075

90105120

135

Specific water uptake rate(cmVcmVcm root/day)

7.S777 X 10"^

11.6274 X 10'^

12.7561 X 10"'

16.6287 X 10"^

19.0183 X 10^'

25.8474 X 10"^

44.8694 X 10"^

50.1098 X 10"'

Root age(d.i\'s)

40

33

30

26

22

1811

5

f(r)

0.15720.23200.25460.33180.37950.51580.89541.0000

J. Agronomy & Crop Science, Vol. 162 (2) 10

138 MALIK, NARDA and MURTY

u c6 \0 20 30

ROOT AGE ( Ar ) A T A N Y SOIL O E P T H , OAYS

1. Variation of root effectiveness function with root age

zone at any time during the wheat growthperiod.Testing of root effectiveness function:For testing of the function f(r) developed inthis investigation, one-dimensional macro-scopic soil water dynamics model underisothermal conditions for homogeneous andnon-deformable soil embedded with root sinkterm incorporating factors viz. f(mc) whichaccounts for the decreased water uptake due todiminishing soil water during the drying cyclesand f(r) which accounts for the decreased rateof water uptake due to root ageing was used as:

66

RLD.

D(9)

RLD,

Az

69

"6^6K(e)

6z

• f(mc) • f(r) (3)

i = 1

Where 9 is the volumetric soil water content(cm^/cm^), z is depth measured positive down-ward from soil surface (cm), t is time (days), K(6) is unsaturated hydraulic conductivity (cm/day), D (6) is soil water diffusivity (cm*/day),Ptr is potential transpiration (cm/day), RLD isroot length density (cm/cm^) at soil depth i, Azis depth increment (cm) and n is the number of

soil depths. The potential transpiration wasestimated by substracting the potential soilevaporation (RITCHIE 1972) from potentialevapotranspiration (DOORENBOS and PRUITT

Table 4: Average moisture contents in the soil pro-file and its variations

Timeaftercrop

sowing(days)

Volumetric moisture content (cmVcm-')

Obser\'ed Simulated

10334251568496

107117

126

0.13870.14150.12790.12230.16580.11840.1311

0.15650,12730.1622

0.1379 (0.58y--0.1443 (1.99)0.1392 (8.84)0.1347(10.14)0.1565 (5.61)0.1331(12.41)0.1452(10.76)0.1590 (1.60)0.1543(21.20)0.1706 (5.18)

Data in parentheses represent percent absolutevariations in simulated moisture contents in rela-tion to the observed.

Development and Testing of Root Effectiveness Function for Soil Water Uptake 139

1977). The function f(mc) developed by MALIK(1985) was used which is a combination ofequal soil water availability at full rate upto aspecified soil water content (0.15 cmVcm^)from field capacity (0.1739 cmVcm"*) and thenlinear decrease to 0.045 times of full rate uptowilting point (0.0523 cm^/cm^). The moisturecontents in the crop root zone at different daysof the wheat growth period were simulatedusing eq. (3) by adopting the solution proce-dure and model input parameters and func-tions as described by MALIK et al. (1988). Thesimulated and field observed soil water con-tents are reported in Table 4.

It has been observed that during the entirewheat crop growth period on an average forthe entire root zone the differences betweenthe observed and simulated soil moistures were7.83 per cent. These differences are mainly dueto the approximations and assumptions used inthe soil water dynamics model and due to theassumptions made for development of functionf(r). These differences are also due to the as-sumption of multiplicative nature of f(mc) andf(r) causing less water uptake by plant roots.For wheat crop grown under normal soil con-ditions with normal irrigation levels, the func-tion f(r), though location specific, can success-fully be used for soil water dynamics studiesunder cropped conditions.

Zusammenfassung

Die Entwicklung und der Test einer Wurzel-leistungsfunktion hinsichtlich der Bo-denwasseraufnahme

Fiir die Bodenwasseraufnahme unter Feldbe-dingungen ist die Wurzeleffizicnz hinsichtlichder Wasserabsorprion von grofSer Bedeutung.Fiir die Entwicklung einer Wurzeleffizienz-funktion wird Information uber das Wurzelal-ter jeweils in einer gegebenen Bodentiefe undzu einer bestimmten Untersuchungszeit wah-rend der Bestandesentwicklung benotigt.Hierfiir wurde eine Funktion entwickelt. Wei-zen wurde als Untersuchungspflanze verwen-det. Es konnte beobachtet werden, dafi dieWurzeleffizienz hinsichtlich der Wasserauf-nahme wahrend der friihen Wachstumsperiodedes Wurzelsystems gleich bleibt, danachnimm: die Effizienz exponenticU mit demWurzelalter ab. Die Anwendung der vorge-

schlagenen Wurzeleffizienzfunktion wurde er-folgreich getestet unter Verwendung eines ma-kroskopischen Modells fiir die Bodenwasser-dynamik, wobei 7,83 % Variation zwischenbeobachtetem und simuliertem Bodenwasser-gehalt auf der Grundlage der Bewurzelungszo-ne des Bestandes fiir die gesamte Anbauperio-de auftrat.

Acknowledgement

This study was conducted as a part of DoctoralResearch on "Development of Root Growth andWater Uptake Model for Wheat". The financial as-sistance provided by the Indian Council of Agricul-tural Research and the physical facilities provided bythe Department of Soil and Water Engineering, Pun-jab Agricultural University, Ludhiana, India, aregratefully acknowledged.

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140 MALIK et al.. Development and Testing of Root Effectiveness Function for Soil Water Uptake

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