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SPARSE CANOPY SYMPOSIUM INTRODUCTION

Soil Type, Climatic Regime, and the Response of Water Use Efficiency to Crop Management

Dep. of Soil Science, The Univ. of Reading, P.O. Box 233, Whiteknights, Reading, RG6 6DW, UK

p.j.gregory{at}reading.ac.uk

	ABSTRACT

TOP ABSTRACT INTRODUCTION A conceptual framework Experimental results Crop intensification, soil type,... Conclusions REFERENCES

 
In many rainfed regions of the world, only a small fraction of the total water available for crop production is transpired and water use efficiency (WUE) is low. Changes in crop management practice to reduce evaporation from the soil surface (E_s) have been successful in some locations but unsuccessful elsewhere. This paper outlines a conceptual framework for assessing the potential for improved crop management to reduce E_s and summarizes results from Syria, Kenya, and Niger. The results show that factors such as evaporative demand, amount and frequency of rainfall, soil texture, and the distribution of roots interact to influence the sensitivity of E_s to management practices that modify canopy area and root growth. Using a simulation model we demonstrate the quantitative effect of these interactions and show that the scope for reducing E_s is greatest in clay soils in locations with frequent rain and low evaporative demand and least on sandy soils in regions with sporadic rainfall and high evaporative demand. The distribution of roots has a marked influence on the rate of drying of the soil surface and thereby on soil hydraulic conductivity which becomes more important as evaporative demand increases relative to rainfall.

Abbreviations: WUE, water use efficiency

	INTRODUCTION

TOP ABSTRACT INTRODUCTION A conceptual framework Experimental results Crop intensification, soil type,... Conclusions REFERENCES

 
WATER is the major factor limiting crop yields in many regions of the world. Even where water for irrigation is currently plentiful, there are increasing concerns about future availability (Falkenmark, 1997). The restricted availability of resources has focussed attention on the efficiency with which water is used, especially in the production of crops where growth and transpiration are intimately connected (Bierhuizen and Slatyer, 1965; Tanner and Sinclair, 1983). Many workers have commented on the small proportion of water potentially available to crops that is actually transpired in some environments. For example, Allen (1990) measured evaporation and evapotranspiration of barley (Hordeum vulgare L.) crops in northern Syria and, by difference, found that transpiration was a small component of total water use by both fertilized (33%) and unfertilized (23%) crops. Similarly, in Niger where rainfall frequently occurs as intensive showers and soils are sandy, evaporation from the soil surface normally exceeds transpiration (Wallace et al., 1993) and drainage is, in many cases, almost equal to the water lost as evapotranspiration (Gaze et al., 1997). Such findings have led many to conclude that the efficiency with which water is used to produce crops could be significantly improved in many rainfed environments (Siddique et al., 1990; Payne, 1997).

Agronomists frequently evaluate the effects of species, genotype, and management in terms of water use efficiency (WUE) written as:

(1)

Both the numerator and denominator can be specified in many ways. If the term "water used to produce yield" is defined as the sum of evaporation from the soil surface (E_s), transpiration (T), net runoff (R) and drainage below the root zone (D), then manipulation of Eq. [1] allows the WUE to be defined in terms of the biomass (N) produced per unit of water lost from the soil profile:

(2)

As Gregory (1989) has noted, this equation makes clear that improved WUE (and hence crop dry matter production) can come about by either crop improvement that increases N/T or agronomic management practices that maximize T by reducing the other losses (i.e., water conservation). Increasing the total amount of water available to a crop (by, for example, irrigation) may increase crop yield but will only increase WUE if T is increased proportionately more than [E_s + R + D] (Gregory, 1989).

Physiological traits suggested for increasing N/T include early vigor, glaucousness, and carbon isotope discrimination (Loss and Siddique, 1994) although specific incorporation of these traits into breeding programs has rarely occurred. However, traditional crop improvement programs have noted small increases in WUE. For example, Siddique et al. (1990) found WUE increased from 23.5 to 28.3 kg/ha mm for wheat varieties released in 1860s and 1982. Much greater improvements in WUE have been indicated through changes in crop management practices (crop intensification) including better weed control, mulching, surface cultivation, modifying plant population, and application of fertilizers (Cooper et al., 1987b; Wallace and Batchelor, 1997). Many such practices have aimed to reduce the fraction of the incident radiation reaching the soil surface and thereby reduce E_s. However, practices that have proved successful in one location have been unsuccessful elsewhere. For example, in northern Syria, Cooper et al. (1987a) found that E_s was reduced by rapid canopy expansion that reduced radiation transmitted to the soil surface, but Yunusa et al. (1993) in Western Australia found that E_s was relatively insensitive to transmitted radiation. Similarly, the large effects of fertilizer applications on growth and WUE in Syria (Cooper et al., 1987b) were not translated to Kenya (Pilbeam et al., 1995).

The purpose of this paper is to explore the effects that soil properties and climatic regimes have on the potential to increase WUE through management practices that modify canopy and root growth. We establish a conceptual framework in which to explore the potential role of crop management and briefly summarize past results in three semiarid countries with contrasting soil and climatic conditions (Syria, Kenya, and Niger). Finally, we use a model to explore the interactions between rainfall, atmospheric demand for water, and the hydraulic conductivity of the upper soil and develop notions of how WUE might best be improved under specified circumstances.

	A conceptual framework

TOP ABSTRACT INTRODUCTION A conceptual framework Experimental results Crop intensification, soil type,... Conclusions REFERENCES

 
Equation [2] demonstrates that WUE can be increased by increasing T at the expense of other losses from the rooting zone (E_s, R, and D). In this paper we concentrate on crop management practices that modify the growth of either the crop canopy or root system or both to reduce E_s, and exclude soil management practices that modify the soil surface such as tillage or mulching. A framework is required for assessing the potential of improved crop management to reduce evaporation from the soil surface. The presence of a crop can reduce E_s by several mechanisms:

1. The presence of a leaf canopy will reduce the net radiation absorbed by the soil surface.
   
2. The presence of a leaf canopy will humidify the air, and increase the aerodynamic resistance to the transfer of water vapor away from the soil surface.
   
3. Uptake of water from near-surface soil will reduce the soil hydraulic conductivity, and thereby restrict the upward flow of water through the soil matrix.
   

When the soil surface is wet, the principal mechanism by which crops reduce E_s is undoubtedly the reduction in the supply of radiant energy to the soil surface. Many models which incorporate an element for partitioning E_s and T assume that the potential evaporation rate is partitioned between potential E_s and potential T based on the fraction of solar radiation that is transmitted through the canopy to the soil surface (e.g., Ritchie, 1972; Cooper et al., 1983). This fraction is usually predicted using the Beers-Lambert Law with an appropriate extinction coefficient. However, even in this relatively straightforward case, a recent analysis by Pearson et al. (1999) has shown that a Beers-Lambert Law partitioning of radiation might be misleading, because it does not take account of exchanges of radiation between the soil and canopy. Pearson et al. (1999) demonstrated that the fraction of net radiation allocated to the canopy (and hence the partitioning of potential evaporation) could be considerably greater than would be predicted from the Beers-Lambert Law using an extinction coefficient derived from measurements of the transmission of solar radiation. Figure 1 shows that while a Beers-Lambert Law approach to partitioning radiation would reduce the radiant energy reaching the soil surface from 0.78 to 0.08 as leaf area index increased from 0.5 to 5.0, the new analysis indicates a reduction from 0.6 to 0.15. Increasing the leaf area index from 0.5 to 5.0 will, then, reduce the potential E_s by 45% rather than the 70% implied by earlier analyses.

View larger version (13K): [in this window] [in a new window]   Fig. 1 Comparison of estimates of the fraction of net radiation available at the soil surface during the day using the Beers-Lambert Law (no symbols) and the model of Pearson et al. (1999)—with symbols. The leaf area index was either 0.5 (solid lines) or 5.0 (dashed lines)

	Experimental results

TOP ABSTRACT INTRODUCTION A conceptual framework Experimental results Crop intensification, soil type,... Conclusions REFERENCES

 
Experiments in three environments have been selected to demonstrate the variability in responses of experiments designed to manipulate canopy size and the components of the water balance. In all cases, T was obtained from the difference between total evaporation and the amount evaporated directly from the soil surface. E_s was estimated by a combination of either direct measurement with microlysimeters or the use of simulation models verified in the appropriate environment.

Winter Rainfall, Low Evaporative Demand, Clayey Soils
Several experiments were conducted over 8 yr at Breda (35°55'N, 30°10'E) in northern Syria (mean rainfall 280 mm) on a Typic Calciorthid (28.8% clay at 0–20 cm; about 40% clay thereafter to 1.5 m). The region has a Mediterranean climate with winter rainfall of large variability both within and between seasons. Typically rain falls between October and late April with most rain in a small number of intense falls >2 mm although, at the nearby city of Aleppo, rainfall <2 mm fell on almost half of the rainy days (Dennett et al., 1983). Potential evaporation during the cool winter is 0.5–1 mm d^-1 and the mean daytime saturation deficit is 0.3 kPa. Both increase rapidly during March and April to values of 5–6 mm d^-1 and >1.2 kPa, respectively. On these soils, rainfall infiltrates rapidly and there is no runoff except on sloping land with shallow soil. Moreover, there is little or no drainage because the soils are deep, and the storage capacity in the rooting zone is greater than the seasonal rainfall.

Applications of P and N fertilizers increased shoot dry matter production of barley significantly (see, e.g., Cooper et al., 1987a for full analysis) and altered the balance between T and E_s (Table 1) . A major influence of fertilizer was to promote early growth and reduce the transmission of radiant energy to the soil surface. For example in 1986/1987, interception of light by crop canopies increased from 0.04 in early January to 0.3 in late March in crops with no fertilizer but from 0.07 to 0.57 at the same times in crops given N and P fertilizers (Allen, 1990). Simultaneously, E_s was reduced and the proportion T/(E_s + T) was always greater in crops given fertilizer. Because there was no drainage, E_s + T corresponds to the water used by the crops and there was no significant effect of fertilizer on this quantity. The overall effect, then, appeared to be that fertilizer increased shoot dry matter production, leaves shaded the soil surface and changed the partitioning between E_s and T, and WUE was increased.

Bimodal Rainfall, Medium Evaporative Demand, Clay Loam Soil
Three agronomic management practices (application of N fertilizer, plant population and irrigation) were examined over four growing seasons at Kiboko (2°10'S, 37°40'E), Kenya. The soil was an acriorthic Ferralsol (a sandy clay loam overlying a sandy clay and well drained). The site has a bimodal rainfall distribution with an annual average of 600 to 700 mm and, typically, >70% of the rain fell on <30% of the rainy days. Potential evaporation was 3 to 5 mm d^-1 on non-rainy days and drainage was rarely >10% of rainfall and often considerably less. Full details of the experiments are given by Pilbeam et al. (1995).

Table 2 shows that T/(E_s + T) increased from an average of 0.14 to 0.35 as rainfall increased from 158 to 475 mm; overall, though, >65% of the water used was not transpired. Within a single season, average values of T/(E_s + T) were similar for maize (Zea mays L.), bean (Phaseolus vulgaris L.) and cowpea [Vigna unguiculata (L.) Walp] crops except in the short rains of 1991 when values for maize were greater than those for bean in the plant population experiment but not in the irrigation experiment. Seasonal rainfall, then, was the largest determinant of T/(E_s + T) and for any given season, a change in agronomic practice only altered T/(E_s + T) by <4% for any crop (Table 2). There was no consistent effect of either N fertilizer or plant population on T/(E_s + T). For example, as N application to beans increased from 10 to 120 kg ha^-1, T/(E_s + T) increased in the short rains of 1990 but decreased in the long rains of 1991; the opposite result was found for maize. Similarly, in the plant population experiment, low plant density resulted in greater T/(E_s + T) in beans but a lower value in maize.

Summer Rainfall, High Evaporative Demand, Sandy Soils
Pearl millet crops were grown at the ICRISAT Sahelian Centre, Sadoré (13°15'N, 2°17'E) in Niger in two seasons. In the first season, traditionally managed crops were compared with a more intensive production system involving an improved variety, denser planting, and application of N and P fertilizers. In the second season, an intensive production system with very high planting density was used (see Daamen et al., 1995 for details). The soil was a Psammentic Paleustalf (the A horizon has 91% sand and 4% clay) with a deep profile (about 5 m). Rain falls between June and mid-September in a series of intense storms and potential evaporation on non-rainy days is high (typically 3–5 mm d^-1; Wallace et al., 1993). Typically, E_s is 35 to 45% of seasonal rainfall in such environments (Wallace and Batchelor, 1997).

The improved agronomy reduced E_s by only 12% relative to E_s in the traditionally managed crop in the first season and by16% relative to bare soil in the second season. However, the total shoot mass produced by the improved treatment (3.4 t ha^-1) was almost twice that of the traditional crop (1.9 t ha^-1). Significant reductions in E_s from the more intensively managed crops were measured in a limited period (mainly in August) when there was a large difference in leaf area (>0.5). However, Daamen et al. (1995) concluded that the effects of the canopy on the amount of radiation incident on the soil surface and on the aerodynamic transfer of water away from the soil surface were small compared with the effect of greater drying of the near surface by root water uptake. They also noted that, in contrast to the studies in Syria, the differences in the rate of E_s occurred during the water limited stage of evaporation implying that root uptake limited the supply of water to the surface. Crops that extract water from near the soil surface are, in effect, competing for soil water with E_s and, in this case, dominate the overall reduction of E_s.

In such environments, improved agronomic management may have a greater influence on D than E_s. This is demonstrated by an analysis of drainage beneath millet crops grown on sandy soils in the Sudano-Sahelian zone (Gaze, 1996; Gregory et al., 1997). For several studies conducted on farmer-managed fields with no, or very low, fertilizer inputs and low plant populations, about 80% of the seasonal rainfall >240 mm was lost by drainage. In contrast, results from experiments at research centers on similar soils where the crops were more intensively managed showed much less drainage. In a specific study on a site adjacent to that described above, Gaze et al. (1997) determined that once there was sufficient infiltration to cause drainage, <30% of the additional rainfall was lost as either T or E_s.

	Crop intensification, soil type, and water use efficiency

TOP ABSTRACT INTRODUCTION A conceptual framework Experimental results Crop intensification, soil type,... Conclusions REFERENCES

 
As demonstrated by the conceptual framework and the results, factors such as evaporative demand, frequency of rainfall, soil texture, and the distribution of roots interact in a complex way to influence the sensitivity of E_s to management practices which affect the size of the crop canopy. We now attempt to examine these interactions using a simple modeling approach. To reduce the problem to manageable proportions, the approach adopted has been to use a simulation model to predict, for several simple scenarios, the effect on E_s of changing the potential rate of E_s from 0.4 to 0.8 of the overall potential evaporation rate. This is approximately equivalent to increasing leaf area index from about 1 to 3 for many cereals.

The model used for this analysis was SWIM (Ross and Bristow, 1990). Previous experience with using SWIM to simulate the partitioning of E_s and T has shown that the results obtained are comparable with more complex models [such as SiSPAT, Braud et al., (1995) and SWEAT, Daamen and Simmonds (1996)], and are consistent with field measurements of E_s using lysimetry provided that certain criteria are followed (Gaze, 1996). A key criterion is that the model is run using subdaily timesteps to simulate the restriction to E_s that can occur during periods of high evaporative demand when E_s becomes water-limited rather than energy-limited. Gaze (1996) used field measurements on a sandy soil in Niger and found that the impact of diurnal fluctuation in evaporative demand on E_s could be accounted for by running SWIM with 6 h timesteps. Daily potential evaporation was distributed between the four quarters of each day according to the ratio 0:0.25:0.5:0.25.

The model simulated a 20 d period during which the amount of rainfall was set to be equal to the potential evaporation. The scenarios simulated were all combinations of the properties shown in Table 3 . The `crop cover' parameter is a slight misnomer, as it is used within SWIM to calculate the fraction of the potential evaporation that is allocated to potential E<sub>s</sub>. In principle, this is equivalent to the partitioning of net radiation between soil and canopy surfaces, which is similar to (but not identical to) the percentage of incident solar radiation transmitted to the soil surface. Hence in the context of this paper, the potential E<sub>s</sub> associated with a given `crop cover' (Eq. [3]) should be interpreted as the rate of E_s that would be achieved if both the soil and canopy surfaces were freely evaporating, and is therefore dependent on the crop cover. Following each simulation, the cumulative E_s was noted.

(3)

The merit of calculating in this way is that it normalizes with respect to the prevailing evaporative demand, and, more importantly, can be shown to be relatively stable over a wide range of changes in ground cover. Hence the conclusions reached about the sensitivity of E_s to increasing the ground cover are generally applicable. A value of 1 for I indicates that the saving in E_s brought about by increasing ground cover is that which would be expected if the saving was equal to the reduction in potential evaporation from the soil surface. A value of <1 indicates that the saving in E_s resulting from increased ground cover is less than would be expected from the reduction in potential soil evaporation. Such values arise in situations where E_s is limited in part by the availability of water, rather than the supply of energy. In such circumstances, it might reasonably be expected that the effect of the crop on E_s would be influenced by the distribution of roots.

Figure 2 shows the results of the simulations for the three soil types. The clearest responses to the various combinations of factors are evident for the sand. In all cases, rewetting the soil every 2 d maintained the surface sufficiently wet for the rate of evaporation from the soil surface to be unrestricted by the supply of water (hence ). For the sandy soil, extending the interval between rain allowed the soil surface to dry out between rain events such that E_s was limited by the supply of water. In such cases, there is less scope for reducing E_s by increasing ground cover (i.e., I fell substantially below 1). This effect was even more marked for scenarios with the higher evaporative demand. The distribution of roots had a pronounced effect. In the `shallow root' simulations, there was much more rapid uptake of water by roots from soil close to the surface, which had the effect of `throttling' the water supply available for direct evaporation from the soil surface. One of the consequences of this was to reduce substantially the scope for reducing E_s by increasing the size of the canopy. In the most extreme case (i.e., with sandy soil, high evaporative demand and 20 d rainfall interval), the potential saving of E_s by manipulating ground cover was <20% of that when the soil surface was continuously wet. The same pattern was evident for the other soil types, though it was much less marked. Indeed, the evaporation from the clay loam in the low evaporative demand environment remained highly susceptible to changing ground cover, even when rainfall events were 20 d apart.

View larger version (15K): [in this window] [in a new window]   Fig. 2 The effect of soil type, rainfall distribution and root distribution on the index I. Simulations used either shallow roots and rainfall at 2 mm d-1 (•) and 6 mm d-1 (), or deep roots and rainfall at 2 mm d-1 () and 6 mm d-1 ()

While the index I indicates the trends that would be expected in moving from one environment to another, the absolute values will depend strongly on factors such as the amount and distribution of rainfall in relation to the potential evaporation. Figure 3 shows how I responds to decreasing amounts of rainfall in an environment with 6 mm d^-1 potential evaporation and a 10 d rainfall interval. Clearly, as rainfall is reduced relative to evaporative demand so the scope for influencing E_s by manipulating ground cover is reduced. Interestingly, the size of the effect is similar irrespective of soil type. This suggests that the smaller the rainfall relative to the potential evaporation, the smaller is the scope for altering E_s and thereby influencing T/(E_s + T) and WUE. The contrasting results obtained at Breda, Syria and Kiboko, Kenya confirm the conclusion of this simulation analysis.

View larger version (12K): [in this window] [in a new window]   Fig. 3 The effect of rainfall on the index, I, for a clay loam (•), sandy loam (), and sand ()

	Conclusions

TOP ABSTRACT INTRODUCTION A conceptual framework Experimental results Crop intensification, soil type,... Conclusions REFERENCES

1. The scope for reducing E_s is greatest on clayey soils with frequent rain showers and low evaporative demand and least on sands with infrequent rain and high evaporative demand.
   
2. Drying of the soil surface by roots is an important means of reducing E_s especially when the evaporative process is water limited. The distribution of roots had least effect on E_s in the clay loam because its higher unsaturated conductivity at modest suctions meant that there was much less time spent under conditions when E_s was water-limited, and therefore susceptible to the effects of soil drying by abstraction of water by roots.
   
3. Issues of "trading-off" are important and require more research. For example, if E_s is reduced to maximize T then within canopy humidity may be reduced with adverse consequences for N/T. Moreover, if increasing WUE results in reduced D in arid regions then reduced groundwater recharge may influence the availability of drinking water.
   

Finally, there may be other reasons for adopting measures to improve crop management and production besides increasing WUE.

	ACKNOWLEDGMENTS

 
We are grateful to the Department for International Development of the UK who funded most of the field-based research. The soil evaporation modelling was carried out as part of Natural Environment Research Council project GR3/10853.

Received for publication September 15, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION A conceptual framework Experimental results Crop intensification, soil type,... Conclusions REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (13) Citing Articles via Google Scholar Google Scholar Articles by Gregory, P. J. Articles by Pilbeam, C. J. Search for Related Content PubMed Articles by Gregory, P. J. Articles by Pilbeam, C. J. Agricola Articles by Gregory, P. J. Articles by Pilbeam, C. J. Related Collections Agroclimatology Water Use Other Crop Management Other Models Other Soil Types