Water scarcity is often a major limiting factor in cotton ( Gossypium hirsutum L.) production, and sustaining productivity and profitability with limited water is a major challenge for the cotton industry. A good understanding of the magnitude, timing and spatial distribution of cotton soil water extraction is important for proper irrigation management, and for development of accurate crop models and decision support systems. The overall objective of this study was to evaluate the water extraction distribution of cotton under different irrigation regimes. Specific objectives were to quantify: 1) the depth of soil water extraction as a function of time, 2) the percent of seasonal water extraction from each soil depth, and 3) the relationship between depth of soil water extraction and canopy height. To meet these specific objectives, daily and seasonal cotton soil water extraction were determined from continuous records of water content in the soil profile measured from four irrigation treatments during a field experiment. We found that cotton extracted soil water from as deep as 150 cm, but the percent of seasonal extraction sharply decreased with soil depth. The top 50 cm soil layer accounted for 75% of the seasonal extraction and the top 80 cm, for 90%. We also found that from 32 days after sowing (DAS) to 100 DAS, the depth of soil water extraction increased linearly at a rate of 1.89 cm ·day -1 or 2.36 times the increase in crop canopy height. These findings suggest that cotton producers should manage irrigations to maintain adequate moisture in the top 80 cm of the soil profile rather than relying on moisture stored deeper in the profile.
Water scarcity is often a major limiting factor in cotton (Gossypium hirsutum L.) production. Therefore, how to sustain or increase productivity and profitability with limited water is one of the biggest challenges facing the cotton industry in many areas of the world. This requires increasing the beneficial use of water, which implies producing more crop quantity and quality with the same amount or even less water. Some workers refer to this concept as to increase crop water productivity. According to [
Improving the beneficial use of water requires improving water management at different scales, including the basin, the farm and the field. At the field scale, one of the important issues includes knowing how much water to apply and when to apply it, commonly referred to as irrigation scheduling. If water is limited, then it is important to know the impact of crop stress at different times during the season, so that irrigation is applied when benefits to the crop are maximized and/or negative impacts are minimized. To be able to properly manage water under limited water situations, it is therefore important to know how crops subjected to different water regimes use soil water. Some of the relevant questions include: how much water is extracted? When is water extracted?, and from which soil depths is water extracted? A good understanding of the magnitude, timing and spatial distribution of cotton soil water extraction is important for proper irrigation management, and for development of accurate crop models and decision support systems.
Several models have been proposed to explain the growth of the effective rooting depth and the rate of extraction of soil water by the crop. For example, [
Z = Z o + ( Z x − Z o ) ( t − t o 2 ) ( t x − t o 2 ) n (1)
where, Z = effective rooting depth at time t (m), Zo = sowing depth (m), Zx = maximum effective rooting depth (m), to = time to reach crop emergence (days or growing degree days [GDD]), tx = time after planting when Zx is reached (days or GDD), t = time after planting (days or GDD), and n = shape factor, which is crop-specific and determines the decreasing speed of the root zone expansion in time. They also suggested procedures to limit root expansion when the crop is water-stressed and for crops growing in shallow soils. This model suggests that during the expansion period, after a lag period, the growth of Z may or may not be linear, depending on the crop. For cotton, they suggested values of minimum effective rooting depth (Zn) of 0.30 m, Zx of up to 2.80 m, n = 1.5, and root expansion rate of 1.5 - 2.5 cm∙d−1. The n ≠ 1 for cotton suggest that the increase in Z for this crop is non-linear.
Also, [
θ = θ l + θ a exp [ − k l ( t − t c ) ] (2)
where, θ = volumetric soil water content (cm3 cm−3), θ1 = lower limit extractable water content (cm3∙cm−3), θa = maximum amount of water that roots can extract from surrounding soil (cm3∙cm−3), k = constant relating to the diffusivity of water flow to and through the roots (cm2 day−1), l = root length density (cm of roots per cm3 of soil), t − tc = duration (days) of the exponential decay, which starts in a soil layer at time tc. The extraction rate (cm3∙cm−3∙day−1) can be obtained by taking the derivative of Equation (2) with respect to time as:
− d θ / d t = k l θ a exp ( − k l ( t − t c ) ) = k l ( θ − θ l ) (3)
Equations (2) and (3) apply for t > tc. For t ≤ tc, θ = θa and dθ/dt = 0.
This last model is commonly used in Australia [
The overall objective of this study was to evaluate the water extraction distribution of cotton grown under four irrigation regimes. Specific objectives included evaluating: 1) the depth of soil water extraction as a function to time, 2) the % of seasonal water extraction from each soil depth, and 3) the relationship between depth of soil water extraction and canopy height.
Data for this study were collected from a field experiment conducted during the 2007-08 cotton season at the Kingsthorpe research station of the Department of Agriculture and Fisheries. The station is located in a sub-tropical climatic zone, about 20 km north-west of the city of Toowoomba, Queensland, Australia (27˚30'44.5'' Latitude South, 151˚46'54.5'' Longitude East, 431 m above mean sea level). The soil at the site is a Haplic, self-mulching, black, Vertisol of alluvial fan and basalt rock origin, slowly permeable, and with a surface slope of about 0.5%. It has a heavy clay texture in the 1.5 m root zone profile, with a distinct change in soil colour from brownish black in the top 90 cm to dark brown deeper in the profile. Physical properties of the soil profile are shown in
The field experiment had four irrigation treatments and three replications arranged in a randomized complete block design. Each experimental plot was 13 m wide × 20 m long, with the crop planted in the North-South direction. A buffer strip (4 m wide) was allowed between plots and a road (4 m wide) was located at the centre of the research area. The irrigation treatments included a fully-irrigated (T50%), deficit-irrigated 1 (T60%), deficit-irrigated 2 (T70%), and deficit-irrigated 3 (T85%) treatment, for which irrigation was applied when 50%, 60%, 70%, or 85% of the plant available water capacity (PAWC) was depleted, respectively.
The cotton hybrid Sicala 60 BRF, which is a Bollgard® II Roundup Ready Flex® variety, was planted on 12 Nov 2007, still within the Bollgard® II cotton planting window for the Darling Downs, which extended from 15 Oct to 26 Nov.Sicala 60 BRF is classified as a medium maturity variety with very good yield potential for late planting [
Fertilizer applications included 188 kg∙ha−1 of starter fertilizer (10.5% N - 19.5%P - 0%K - 2.2%S) and 126 kg∙ha−1 of granular Urea (46% N) applied at sowing (12 Nov), and an additional 190 kg ha−1 of Urea applied on 18 Jan.
Soil Depth (cm) | Bulk Density (g∙cm−3) | Coarse Sand (%) | Fine Sand (%) | Silt (%) | Clay (%) |
---|---|---|---|---|---|
0 - 10 | 0.89 | <1 | 8 | 17 | 76 |
40 - 50 | 1.03 | <1 | 7 | 18 | 76 |
60 - 70 | 1.05 | 2 | 8 | 19 | 72 |
100 - 110 | 1.07 | 2 | 10 | 17 | 73 |
120 - 150 | 1.08 | 3 | 7 | 17 | 73 |
Weeds were controlled by a combination of manual chipping (3 Dec), mechanical cultivation (9 Jan), and a chemical control using Roundup (15 Jan). Main weeds were the Dwarf amaranth (Amaranthus macrocarpus) and Tarvine (Boerhavia dominii). The insecticide Decis (Deltamethrin) was applied on 15 March to control the Pale Cotton Stainer (Dysdercus sidae) insect.
The plots were irrigated individually with bore water using a hand-shift sprinkler system, which was fitted with partial-circle sprinkler heads to avoid irrigating adjacent plots. Irrigations were applied during times with low wind speeds to obtain adequate application uniformity. Irrigation depths were measured using a rain gauge installed at the centre of each plot. Irrigations were scheduled based on weekly measurements of soil water content from each plot using the neutron probe method.
The crop was defoliated when it reached four nodes above cracked boll (NACB). Defoliant was applied on 29 - 30 April to all treatments, except for the T50% treatment, which had delayed maturity and received defoliant twice on 7 and 15 May. The crop was harvested on 12 - 14 May, except for the T50% treatment, which was harvested on 22 May. The soil was then tilled twice (27 and 28 May) to comply with Pupae Busting. Pupae Busting was a requirement of the license for planting Bollgard® II cotton varieties in Australia, which mandated the full disturbance of the soil surface to a depth of 10 cm to prevent overwintering of the Helicoverpa.
Soil water content to schedule irrigations was measured weekly using the neutron probe method. A neutron probe access tube was installed in each plot and readings were taken at 10 cm depth increments to a depth of 150 cm with a 503DR Hydroprobe (CPN International, Inc., Martinez, CA, USA), which was calibrated for the soil at the research site against gravimetric soil moisture measurements.
Also, soil water content was automatically monitored every 30 min using EnviroSCAN® Solo (Sentek sensor technologies, Stepney, South Australia) capacitance probes installed in each treatment. This information was used to determine the depth of soil water extraction and the percent of water extraction from each soil depth. The EnviroSCAN® Solo data was used for this purpose, rather than the neutron probe data, because it provided a continuous record of soil water content in the different soil depths, which allowed a more accurate assessment of the time when soil water extraction started from each depth.
Each EnviroSCAN® Solo probe was customized to measure soil water from twelve soil depths, including 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, and 150 cm. According to the EnviroSCAN® Solo manufacturer, each sensor records moisture data from a soil volume outside the access tube with a sphere of influence of 10 cm vertical height and a radial distance from the outer wall of the access tube of 10 cm. The probes were manually installed in the field on 14 Dec 2007 after the crop was established. Installation prior to this time was not possible due to frequent rainfall early in the growing season and the high risk of damaging the small cotton plants.
Prior to field installation, each of the twelve sensors in each probe was normalized by taking “air count” readings with the probe inside the PVC access tube while suspended in air, and “water count” readings with the probe in a water bath or normalization container that was constructed using an ice cooler and a piece of access tube (
Depth (cm) | Air Count | Water Count |
---|---|---|
20 | 36,797 | 26,285 |
40 | 36,661 | 26,242 |
60 | 37,463 | 26,712 |
80 | 37,269 | 26,434 |
100 | 36,686 | 26,318 |
120 | 36,758 | 26,301 |
The raw soil counts from each sensor were converted to volumetric soil water contents as [
S F = F a − F s F a − F w (4)
S F = A θ B + C (5)
θ = ( S F − C A ) 1 B (6)
where, θ = volumetric soil water content (cm3∙cm−3, or mm of water per 100 mm of soil measured), SF = scaled frequencies, Fa = air count, Fs = soil count, Fw = water count. SF, Fa, Fs, and Fw are all unitless and A, B, and C are empirical factors that depend on soil type. We used values of A = 0.0254, B = 1.00, and C = 0.011, recommended by the manufacturer for soils similar to the soil at the research site.
The EnviroSCAN®So1o data for each treatment was visually inspected to determine the time (DAS) when soil water extraction started from each depth. This point has been defined [
In this study, the percent of seasonal water extraction from each soil depth was determined following these steps:
1. The daily soil water extraction for each soil depth was calculated by subtracting the current soil water content to that of the previous day.
2. The daily cumulative water use from each soil depth (cumulative depth ET) and from the whole profile (cumulative profile ET) was calculated.
3. The relative cumulative ET was calculated for each soil depth (relative cumulative EnviroSCAN ET = cumulative depth ET/cumulative profile ET).
4. For each soil depth, the fraction of seasonal water extraction (Seasonal extraction from depth/seasonal extraction from profile) was calculated.
5. For each soil depth, the cumulative fraction of seasonal water extraction was calculated.
Weather variables to characterize the weather conditions during the growing season were measured at the research site using an EnviroStation electronic weather station (ICT International Pty Ltd, Armidale, NSW, Australia), which recorded hourly and daily values of solar radiation (Rs, MJ∙m−2∙d−1), air temperature (˚C) [maximum (Tmax), minimum (Tmin), and average (Ta)], relative humidity (RH, %), wind speed (u, m s−1) and rainfall (mm).
The Tmax and Tmin data were used to calculate daily growing degree days (GDD,˚C day), using “Method 1” of [
GDD = [ ( T max + T min ) / 2 ] − T base (7)
Also,
if [ ( T max + T min ) / 2 ] < T base , then [ ( T max + T min ) / 2 ] = T base (8)
where, Tbase = base temperature (˚C) (temperature below which no crop growth occurs). For cotton, Tbase = 12˚C was assumed. From the daily GDD, the cumulative GDD from sowing (CGDD, ˚C∙day) was then calculated. The weather data was also used to calculate the daily and monthly grass-reference evapotranspiration (ETo, mm) using the Penman-Montheith method as detailed in FAO-56 [
As an indicator of crop development, plant canopy height (h) was measured eighteen times during the season, from soon after crop emergence to defoliation. Measurements were made from four representative plants in each plot, from the soil surface to the top leaf. Also, observations about crop growth stage were recorded throughout the season.
Statistical analysis, plotting, and calculations were conducted with the R language and environment for statistical computing [
Daily weather conditions during the 2007-08 cotton season at Kingsthorpe are shown in
Month | Season | |||||||
---|---|---|---|---|---|---|---|---|
Variable [a] | Nov | Dec | Jan | Feb | Mar | Apr | May | Avg/total |
Tmax (˚C) | 27.0 | 29.0 | 30.7 | 29.2 | 28.0 | 25.4 | 25.0 | 27.8 |
Tmin (˚C) | 14.7 | 17.5 | 17.5 | 16.1 | 12.6 | 7.3 | 3.7 | 12.8 |
Rs (MJ m−2∙d−1) | 24.6 | 22.9 | 22.4 | 22.3 | 24.1 | 20.3 | 19.0 | 22.2 |
RH (%) | 76.3 | 76.5 | 75.2 | 77.4 | 71.7 | 71.6 | 59.1 | 72.5 |
u (m∙s−1) | 2.9 | 2.9 | 3.4 | 2.9 | 3.0 | 2.2 | 1.5 | 2.7 |
Daily ETo (mm) | 4.7 | 4.8 | 5.0 | 4.6 | 4.5 | 3.3 | 2.8 | 4.3 |
Monthly ETo (mm) | 88.5[b] | 149.6 | 156.1 | 132.5 | 140.5 | 100.1 | 36.7 | 804.0 |
Monthly Rain (mm) | 26.0 | 44.0 | 16.0 | 126.0 | 37.0 | 22.0 | 0.0 | 271.0 |
[a]Tmax, Tmin = Maximum and minimum air temperatures, Rs = Solar radiation RH= Relative humidity, u = Wind speed, ETo = Grass-reference evapotranspiration [b]For Nov and May, only data within the cotton growing season was included.
January, which consequently also had the higher monthly ETo. Total rainfall during the growing season was 271 mm, representing only 34% of the 804 mm of seasonal ETo, which explains why irrigation was needed at this site. February was the wettest month with 126 mm of rain, representing almost half (46%) of the seasonal rainfall. The minimum temperature (
Irrigation timing and amounts applied to each treatment are shown in
Observations about crop development stages, and their corresponding dates, DAS and CGDD, are shown in
Crop stress also affected plant canopy height, as shown in
Irrigation Treatment | ||||
---|---|---|---|---|
Date | T50% | T60% | T70% | T85% |
Jan 26, 2008 | 5 | |||
Jan 27, 2008 | 45 | |||
Jan 28, 2008 | 20 | |||
Feb 1, 2008 | 10 | |||
Feb 29, 2008 | 76 | 45 | ||
Mar 4, 2008 | 54 | |||
Mar 27, 2008 | 58 | |||
Apr 21, 2008 | 24 | |||
Apr 22, 2008 | 28 | 28 | ||
Total | 228.0 | 83 | 82 | 0 |
Date | DAS | CGDD (˚C-day) | Crop Stage |
---|---|---|---|
Nov 12, 2007 | 0 | 0 | Sowing |
Nov 20, 2007 | 8 | 73 | Emergence |
Dec 3, 2007 | 21 | 196 | 4 leaves |
Dec 10, 2007 | 28 | 286 | 6 leaves |
Dec 19, 2007 | 37 | 390 | 8 Leaves |
Jan 2, 2008 | 51 | 539 | First Square (9 nodes) |
Jan 25, 2008 | 74 | 813 | 50% Flowering |
Jan 29, 2008 | 78 | 864 | 100% Flowering |
Jan 1, 2008 | 81 | 905 | Crop was fully flowered and some green bolls had developed |
Mar 13, 2008 | 122 | 1303 | A few open bolls in plants with severe water stress |
Apr 4, 2008 | 144 | 1480 | A few open bolls in the fully-irrigated treatment (T50%) |
Apr 30, 2008 | 170 | 1592 | Defoliation of the T60%, T70% and T85% treatments |
May 7, 2008 | 177 | 1605 | Defoliations of the T50% treatment |
May 14, 2008 | 184 | 1620 | Harvest |
DAS = days after sowing, CGDD = cumulative growing degree-days. |
set of crop stress on the deficit-irrigated and dryland treatments. Significant differences started to occur in late January and early February, at about 80 DAS, when the crop was fully flowered. In general, treatments with more irrigation resulted in taller plants, with a treatment plant height ranking of T50% > T60% > T70% = T85%. Cotton plants for the T50% treatments were significantly
taller than the other treatments and continued to grow and to produce new bolls until the crop was defoliated. This is consistent with the fact that cotton is a perennial crop that will continue to grow as long as conditions are favourable.
decrease in opportunity time since it takes more time for roots to reach deeper depths.
extraction after that time. It should be noticed that 100 DAS was also the time when plant canopy height peaked for all treatments, except the T50% treatment which continued to increase in height after that time, but only slightly. This could suggest that at that time the crop root system also stopped growing. Extrapolation of the linear function in
Other researchers have shown that the rate of increase in the depth of extraction, and the maximum depth of extraction, vary with a variety of factors, including crop species, crop variety, soil water content, and growing season. For instance, [
for different sorghum cultivars, with values ranging from 3.27 to 4.92 among cultivars, and lag period of 14.8 - 19.3 days. Similarly, [
For cotton, [
From investigations of soil water extraction from cotton planted in different row configurations, [
The fraction of cumulative EnvironSCAN ETc as a function of DAS for each depth and treatment is shown in
The fraction of seasonal soil water extraction as a function of depth of extraction for each treatment are summarised in
F S W E = 3.147582 e − 1 − 7.327073 e − 3 D + 6.070688 e − 5 D 2 − 1.709402 e − 7 D 3 (9)
where, FSWE = fraction of seasonal water extraction, and D = depth of extraction (cm).
The cumulative fraction of seasonal soil water extraction as a function of depth of extraction is shown in
C F S W E = 6.471062 e − 2 + 2.188146 e − 2 D − 1.847685 e − 4 D 2 + 5.382206 e − 7 D 3 (10)
where, CFSWE = cumulative fraction of seasonal water extraction, and D = depth of extraction (cm).
cm, 75% from the top 50 cm, 90% from the top 80 cm, and 95% from the top 110 cm. Low extraction from deeper depths could be due to a combination of low root density, lower water availability, and insufficient time between the arrival of the extraction front and crop maturity, especially considering that by the time the extraction front reaches the lower depths, the crop water demand (evapotranspiration) had already started to decrease significantly.
Similarly,
irrigation depths were applied rather than refilling the soil profile during each irrigation event. Therefore, the small irrigation depths only tended to wet the top quarter of the crop root zone.
Our findings are consistent with those recently reported by [
These results then suggest that irrigation for cotton should be targeted at wetting only the top 80 cm of the soil profile during each irrigation event, which accounted for 90% of the seasonal water extraction. This means reducing irrigation depths during each irrigation event and increasing irrigation frequently. Currently, the common practice in the Australian cotton industry is to allow relatively high soil water depletions and then applying furrow irrigation to refill the profile to depths far exceeding 80 cm. This common practice is likely to produce crop stress, resulting in lower yields, and water losses by deep drainage and runoff.
Many growers, however, believe that no deep percolation losses occur in the heavy soils common in Australia due to low infiltration rates. However, recent studies have shown that significant losses do in fact occur. For example, [
We found that the depth of soil water extraction for cotton increased linearly with DAS, CGDD, and plant canopy height, from 32 DAS to 100 DAS, until a maximum depth of extraction was reached. The depth of extraction increased almost linearly with DAS and CGDD at a rate of 1.89 cm∙day−1 and 0.165 cm per ˚C day, respectively. The depth of extraction also increased almost linearly at a rate of 2.36 times the crop canopy height during the same period. In this study, however, we were not able to assess the nature of the increase in depth of extraction prior to 32 DAS, before the depth of extraction reached 10 cm. However, the depth of extraction did not start to increase at the linear rate starting at sowing or crop emergence, but there was a lag period that we estimated as 23.5 days. Since depth of extraction is difficult to evaluate in the field, the good linear relationship between depth of extraction and canopy height obtained in this study can be used to assist in irrigation management.
We also found that about 90% of the seasonal water extraction by the cotton crop took place from the top 80 cm soil depth, and depths below 110 cm accounted for only 5% of the seasonal water extraction. The practical significance of these findings is that cotton producers should manage irrigation aimed at maintaining adequate soil water content in the top 80 cm of the soil profile by applying frequent and light irrigations. This contrasts with the current common practice in the Australian cotton industry of applying large irrigation depths to refill the soil profile during each irrigation event, which could lead to large water losses. The information obtained in this study is also valuable for modelling cotton soil water extraction pattern and water use.
The authors would like to acknowledge the contribution provided by the Department of Agriculture and Fisheries, the Cotton Communities CRC, and the Cotton Research & Development Corporation. Technical Contribution No. 6590 of the Clemson University Experiment Station. This material is based upon work supported by NIFA/USDA, under project number SC-1700540. Commercial names were included for the benefit of the reader and did not imply endorsement by the authors or their organizations.
Payero, J.O., Harris, G. and Robinson, G. (2017) Field Evaluation of Soil Water Extraction of Cotton. Open Journal of Soil Science, 7, 378-400. https://doi.org/10.4236/ojss.2017.712027