Sunflower Water Requirements Using Single and Dual Crop Coefficients

Table (2). Some physical and chemical properties of the experimental site

Table (3). Chemical analysis of irrigation water used in the field experiment

At harvest, the sample of plants (1 m of the row  0.60 m width of the row = 0.60 m²) of the two central ridge were chosen to determine the sunflower yield and the total yield per ha^-1 was calculated.

4. Irrigation regime

The irrigation treatments were based on replenishment of soil water depletion according to the reference evapotranspiration (ET₀). The irrigation treatments were:

I1 irrigation at 20% of ET₀,

I2 irrigation at 40% of ET₀,

I3 irrigation at 60% of ET₀,

I4 irrigation at 80% of ET₀, and

I5 irrigation at 100% of ET₀

Irrigation water in drip irrigation system was taken by a water pump. Distribution lines consisted of PVC pipe manifolds for each plot. The diameter of the polyethylene laterals was 16 mm and each lateral irrigated one plant row. The inline emitter discharge rate was 4 l h^-1 at 100 kPa operating pressure. The actual emitter discharge rate was calibrated before starting the experiment. The drip network calibration was performed and the actual rate of emitter was 3.43 l h^-1.

Soil water content was measured by sampling a soil from each row with soil tube 0.025 m diameter at three depths i.e. 0-10, 10-20 and 20-60 cm below soil surface then determined by gravimetric method. Soil water contents were monitored prior each irrigation and after irrigation at surface and subsurface depths through electronic pressure transducer (electronic tensimeter).

5. Crop Evapotranspiration

The irrigation requirements were calculated according to the Penman-Monteith equation (Allen et al., 1998) according the following equation:

                                                                                 (2)

Where:

ET_crop   is the crop evapotranspiration, mm/day

ET_drip   is the crop evapotranspiration under drip irrigation system, mm/day

E_a is the efficiency of irrigation system (assumed as 95 % for drip irrigation system under the present conditions).

LR is the Leaching Requirements required for salt leaching in the root zone depth (assumed as 15 %). and

                                                                                (3)

K_r is the reduction factor that reflects the percent of soil covering by crop canopy and can be calculated by the equation described in Karmeli and Keller (1975):

                                                                                                    (4)

Where, GC is the ground cover fraction (plant canopy area divided by soil area occupied by one plant, assumed as 0.6).

K_c is the crop coefficient ranging from 0.35 (for initial stage) to 1.15 (for development stage) for sunflower (Allen et al., 1998). We need the length and crop coefficient (K_c) for each of the 4 growth stages: initial, crop development, mid-season and late season stages. The crop coefficients (K_c andK_cb) were collected from FAO (Allen et al., 1998) and are presented in Table (4).

Table (4). Crop coefficient (K_c) and development stages period for sunflower

ET₀ is the reference evapotranspiration calculated with FAO Penman- Monteith equation (Allen et al., 1998) using the climatic data collected from the Nouzha Weather Station as follows:

                            (5) 

Where:

The effect of soil water stress on crop ET is accounted by multiplying the crop coefficient by the water stress coefficient (K_s), which is given by the following equation:

                                                                       (6)

Where:

TAW is the total available water in the root zone depth (mm),

RAW is the readily available water in the root zone (mm), RAW=p*TAW,

p   is the fraction of TAW that a crop can extract from the root zone without water stress (assumed as 0.45) and

D_r   is the root zone depletion in the root zone (mm)

The total available water in the root zone is estimated as follows:

                                                                                 (7)

Where:

q_FC is the field capacity (m³/m³),

q_WP   is the permanent wilting point (m³/m³) and

Z_r      is the effective rooting depth (m)

The adjusted K_c due to water stress is:

                                                        (8)

                                                   (9)

The field crop evapotranspiration (ET_C) was calculated using the following equation (10):

                                                                                         (10)

Where ET_C is the crop evapotranspiration (mm), P is precipitation (mm), I is irrigation (mm), D is the water drained (mm), R is the runoff (mm) and DS represents the changes in soil water storage during the growth period. D and R were considered as zero because of control irrigation. The changes in soil moisture were estimated with soil moisture measurements at different depths.

6. Development of Crop Coefficient

The K_c values were developed for sunflower crop using ET₀ estimates from FAO-PM method. To compute K_c based on crop development stage, it is important to establish the length of different crop growth stages (Table 4). Allen et al. (1998) divided the crop cycle into four stages: initial stage (marked with about 10% of plant cover), development stage (marked with the growth of plant 10% ground cover to effective cover i.e., flowering), mid-season stage (effective cover to start maturity) and late season stage (Start of maturity to harvest).

7. Sunflower crop coefficient (K_c)

The crop coefficients of the sunflower during the different growth stages according to the standard FAO methodology were presented in Table (5).

Table(5). Sunflower crop coefficient at growing periods (Doorenbose and Kassam, 1986)

Crop coefficient obtained for four growth stages of crop growing periods. The four growth stages of crop growing periods are as follows:

1. Initial period (planting to 10% ground cover)
2. Crop development (10% ground cover to effective cover i.e., flowering)
3. Mid-season (Effective cover to start maturity)
4. Late-season (Start of maturity to harvest)

The calculation procedure for crop evapotranspiration (ETc) consists of:

1. identifying the crop growth stages, determining their lengths, and selecting the corresponding K_c coefficients;
2.  adjusting the selected K_c coefficients for frequency of wetting or climatic conditions during the stage;
3. constructing the crop coefficient curve (allowing one to determine K_c values for any period during the growing period); and
4.  calculating ET_c as the product of ET_o and K_c.

8. Crop coefficient (field approach)

The single crop coefficient (K_c single) was defined as the ratio of the measured ET_c by field soil moisture measurement to the ET₀ estimated by the FAO Penman –Monteith equation (Allen et al., 1998) under standard condition as follows:

                                                                                                (11)

The dual crop coefficient under standard conditions can be presented as:

                                                                                   (12)

Where:

K_cb is the basal crop coefficient and K_e is the soil evaporation coefficient.

Therefore, crop development and its characteristics were recorded during the growing season to separate the individual growing stages of sunflower being the initial, development, mid and end stages.

9. Crop coefficient (FAO approach)

1. Single crop coefficient

The values for large number of crops are presented in Allen et al. (1998). They are based on average conditions in sub-humid climate. FAO has presented a correction equation to normalize the K_c value for other places with different climatological and soil conditions.

The value of K_{c ini} can be estimated from Figures 29 and 30 (Allen et al., 1998) as follows:

                              (13)

Where:

K_{c ini}   is the value for K_{c ini} from Figure 29 (Allen et al., 1998)

K_{c ini}   is the value for K_{c ini} from Figure 30 (Allen et al., 1998)

I    is the average infiltration depth (mm)

The values 10 and 40 in Equation are the average depths of infiltration(mm) upon which Figures 29 and 30 (Allen et al., 1998) are based.

Drip irrigation wet only a fraction of the soil surface, the fraction of the surface wetted, f_w ranged from 0.3 to 0.4. The Kc_ini can be calculated from the following equation:

                                                                              (14)

Where:

f_w   is the fraction of surface wetted by irrigation (0 – 1), 0.3 for drip irrigation

K_{c ini} (Tab, Fig) is the value of K_{c ini} from Table 12 or Figure 29 or 30 (Allen et al., 1998).

The value of K_{c mid}, specific adjustment in climate where RH_min differ from 45% or where U₂ is larger or smaller than 2.0 m/s was used. The value of K_{c mid} is adjusted as:

                                 (15)

Where:

K_{c mid} (Tab) is the value of Kc mid taken from FAO Table (12), Allen et al. (1998)

U₂ is the mean value of daily wind speed at 2 m height over the soil surface during the mid- season growth stage (m/s) for 1 m/s <= U₂ <= 6 m/s.

RH_min is the mean value for daily minimum relative humidity during the mid-season growth stage 9%), for 20% <= RH_min<=80%.

h   is the mean plant height during the mid-season growth stage (m) for 0.1 m<h< 10 m.

The value of K_{c end} is adjusted as:

                                  (16)

2. Dual crop coefficient

The crop coefficient is divided into two parts (Equation). The first part is the basal crop coefficient (K_cb) that refers to the crop transpiration component of ET_c when the soil surface is dry but transpiration is occurring at a potential rate, i.e., water is not limiting transpiration (Allen et al., 1998). The second part is the soil evaporation coefficient K_e that describes the soil evaporation component of ET_c.

Similar to the single crop coefficient approach, a correction equation is used to determine K_cb in mid- and end-season stages of sunflower through the following equations:

                               (17)

                                (18)

The soil evaporation coefficient (K_e) depends on several parameters such as the irrigation period, irrigation depth, soil properties, wetting area, and crop development.

When the soil is wet, evaporation from the soil surface occurs at maximum rate. Therefore, the dual crop coefficient can never exceed a maximum value, K_{c max}. The K_e can be determined as:

                                                                   (19)

Where:

K_e   is the soil evaporation coefficient (-)

K_cb   is the basal crop coefficient,

K_{c max}   is the maximum value of Kc following irrigation,

K_r   is the evaporation reduction coefficient depends on the cumulative depth of water depleted from the topsoil,

f_ew   is the fraction of the soil that is both exposed and wetted

The K_{c max} range from 1.05 to 1.30 and can be expressed as:

             (20)

                                                                                         (21)

Where:

TEW   is the maximum cumulative depth of evaporation (depletion) from the soil surface layer

REW    is the readily evaporable water (mm)

Di   is the cumulative depth of evaporation (depletion) from the soil surface layer.

                                                                                         (22)

1-f_c   is the average exposed soil fraction not covered by vegetation (0.01-1), Table (6)

f_ew   is the average fraction of soil surface wetted by irrigation (0.01-1)

Table (6). Common values of fractions covered by vegetation (fc) and exposed sunlight(1-f_c), Allen et al. (1998).

10. Experimental design

A randomized complete block design (RCBD) with five treatments. Irrigation treatments were conducted using a drip irrigation system. The drip irrigation system was divided into three plots (replicates), and each plot had one valve.

11. Statistical analysis

Seed and oil yields were analyzed using a single-factor analysis of variance (ANOVA), and multiple comparisons were done for significant effects among treatment with the least significant difference (LSD) test by SPSS (Windows V18). The analysis was performed at 0.05 probability level of significant. The Duncan's Multiple Range Test was used for comparisons among different sources of variance.

 RESULTS AND DISCUSSION

The sunflower growing periods were divided to four stages; initial, development, mid- and late growing stages. The sunflower planting period started on 28 April and was finished on 8 August. Table (7) illustrates the length of growing stages, crop coefficient (K_c) and reference evapotranspiration (ET₀).

Table (7). Growth period, crop coefficient and reference evapotranspiration of sunflower

1. Reference Evapotranspiration (ET₀)

The daily ET₀ was calculated according to the FAO Penman-Monteith equation (Allen et al., 1998). During the sunflower growing season, the daily ET₀ varied from 3.21 to 9.97 mm/day with an average of 4.91 mm/day and total value of 505.4 mm/season. The variation of ET₀ during the growing period is illustrated in Figure (1).

Fig. (1). Daily variation of reference evapotranspiration during growing period of sunflower

2. Crop evapotranspiration (ET_c) of sunflower

1. FAO single crop coefficient (K_c)

The daily sunflower evapotranspiration (ET_c) using standard single crop coefficient is illustrated in Figure (2).

Figure (2). Daily variation of sunflower crop evapotranspiration (ET_c) with irrigation regimes using single crop coefficient.

Table (8) shows the sunflower crop evapotranspiration during initial, development, mid- and late growth stages according FAO standard approach. The crop evapotranspiration was decreased as water regime (% of ET₀) decreased.  The crop coefficient was 0.35, 1.15 and 0.35 for initial, mid- and late growth stages as mentioned by Allen et al. (1998).  

Table (8). Sunflower crop evapotranspiration(mm/ha) of growth stages with irrigation regimes (Single crop coefficient standard FAO approach)

The seasonal sunflower crop evapotranspiration (ET_c) according to standard FAO methodology were 418.5, 344.8, 251.1, 167.4 and 83.7 mm for 100, 80, 60, 40 and 20% ET₀ irrigation regimes, respectively (Table 8).

The seasonal crop evapotranspiration (ET_c) of sunflower according to single crop coefficient field approach were 466.2, 372.9, 279.9, 186.5 and 93.2 mm for 100, 80, 60, 40 and 20% ET₀ irrigation regimes, respectively (Table 9). The single crop coefficient was 0.58, 0.89, 1.19 and 0.35 for initial, development, mid- and late growth stages under field conditions.

There are little differences between standard and field approach of crop coefficient, but the initial crop coefficient (K_{C ini}) is larger in field approach because of field conditions of the present experiment. Generally, crop coefficient depends on weather conditions, growth characteristics and ground cover of sunflower under field conditions.

Table (9). Sunflower actual crop evapotranspiration (mm) of growth stages with irrigation regimes (single crop coefficient field approach)

2. Basal crop coefficient (K_cb)

The crop coefficient, soil evaporation and dual daily crop coefficient of sunflower crop were obtained during the growing period. The values of basal crop coefficient during sunflower growing period are shown in Table (10). The values were 0.32, 0.69, 1.05 and 0.25 for initial, development, mid- and late growth stages according to FAO standard approach (Table 10).The basal crop coefficient (i.e., transpiration component) gradually increased as the highest value was obtained in the development growth stage. Thus, the transpiration value was decreased during late growing stage. The soil evaporation was differed according to the water regime, it reached 0.61, 0.55, 0.51, 0.43 and 0.34 for 100, 80, 60, 40 and 20% of ET₀. The seasonal crop evapotranspiration (ET_c) of sunflower according to dual crop coefficient standard approach were 484.7, 377.7, 291.0, 184.4 and 99.0 mm for 100, 80, 60, 40 and 20% ET₀ irrigation regimes, respectively (Table 10).

Table (10). Sunflower crop evapotranspiration (mm) of growth stages with irrigation regimes (dual crop coefficient standard FAO approach)

The seasonal crop evapotranspiration (ET_c) of sunflower according to dual crop coefficient field approach were 496.1, 388.9, 288.8, 188.7 and 100.8 mm for 100, 80, 60, 40 and 20% ET₀ irrigation regimes, respectively (Table 10). The dual crop coefficient (field approach) was 0.35, 0.69, 1.09 and 0.35 for initial, development, mid- and late growth stages (Table 11).

Table (11). Sunflower crop evapotranspiration (mm) of growth stages with irrigation regimes (dual crop coefficient field approach)

The seasonal crop evapotranspiration (ET_c) of sunflower according to Irrigation field approach was 496.0, 398.1, 299.6, 200.1 and 103.3 mm for 100, 80, 60, 40 and 20% ET₀ irrigation regimes, respectively (Table 12). The crop coefficient was 0.51, 0.90, 1.50 and 0.54 for initial, development, mid- and late growth stages under field conditions.

Table (12). Sunflower crop evapotranspiration (mm) of growth stages with irrigation regimes (Field Irrigation approach)

The daily sunflower crop coefficient can be calculated by the best fitted polynomial equation (Table 13):

Where DAP is the days after planting

Table (13). Crop coefficient during growth stages according to different approaches

The results indicated that seasonal sunflower evapotranspiration (mm) has higher value with field irrigation approach and the lower value was for standard FAO single approach. The seasonal evapotranspiration (single crop coefficient approach) was less value than the seasonal evapotranspiration of dual crop coefficient approach.

The seasonal water requirements for sunflower crop with considering the irrigation and soil practices are illustrated in Table (14). The results indicated that water requirements of sunflower growing season were higher with irrigation approach and lower with single crop coefficient approach.

Comparison of the measured single crop coefficient with standard values of FAO showed that, the measured K_c value at the initial stage was higher than the FAO standard value (by about 74.3% higher). The K_c-ini greatly depends on the evaporating power of the atmosphere (ET₀), the water supply during a wetting event and the time interval between wetting events. Consequently, the K_c-ini is influenced by the different irrigation strategies and soil practices.

 Therefore, field management in the present study may not similar to the FAO-56 conditions. The FAO’s predicted K_c may not predict the evapotranspiration that occurs in the initial growing stage. The measured value of late stage (K_c-end) is larger than proposed value of FAO-56 by about 11.42% (Table 15).

Table (14). Sunflower water requirements (m³/ha) with irrigation regimes

Table (15). Seasonal water requirements of sunflower (%) as related to single crop coefficient (FAO standard)

According to FAO-56 method corrected by equation (16 and 17), the sunflower K_cb values were 0.32, 1.05 and 0.25 for initial, mid- and late-season stages, respectively. Actually, the measured values of K_cb (0.35, 1.09 and 0.35, respectively) were similar to the standard FAO method values. The K_cb values are correlated with crop variety, cultivation pattern, crop coverage, soil practices and also the final crop yield. Different field treatments especially short irrigation intervals may keep the soil water content at optimum or higher value may lead to more or less evaporation occurring that affect the K_e and K_cb values. The field measurement to predict soil evaporation needs some practices to be more accurate to reduce the measured error.

The soil evaporation, K_c and K_cb coefficients are greatly affected by irrigation strategy, canopy coverage, local weather conditions, soil practices and irrigation system, therefore more investigation must be considering in determination of these parameters.

The higher values of sunflower water requirements for dual than single crop coefficients by about 3.1% may be due to more parameters affected the determination of dual K_cb than single K_c. Therefore, the values of K_c must be determined for different regions and different agricultural parameters, then local determination of crop coefficient has been recommended. The water requirements of sunflower with field irrigation approach were more than the dual crop coefficient approach by about 3.48% as mean of all water regimes.

The use of crop coefficients presented by FAO-56 (Allen et al., 1998) is common for use with crop water requirements estimation around the world. The present study showed that dual crop coefficient approach is located between the single crop coefficient and field irrigation approaches ( 3.2%). Therefore, dual crop coefficient is the more precise estimation of crop water requirements of sunflower than single coefficient and field irrigation approaches. The presented values of single and dual K_c will be useful in estimating sunflower water requirements of different crop growth stages and irrigation scheduling under semi-arid regions such as the present experimental conditions.

 It appears that ET_c estimation of sunflower crop is more accurate by dual crop coefficient approach than those produced by single crop coefficient approach because of using more parameters and taking the soil practices and crop characteristics in consideration. The basal crop coefficient values cannot be proposed for all climates and regions because of different climatic conditions and crop management practice under different regions.

The present study recommended that under the same conditions, the irrigation of sunflower crop must be done according to the dual crop coefficient approach because it is more accurate than single crop coefficient and close up to the field irrigation conditions. Also, the field measurement to predict soil evaporation needs some practices to be more accurate to reduce the measured error.