Results and discussionTop

Variations in actual evapotranspiration (ET_c-act), plant transpiration (T_p) and soil evaporation (E)

The values for ET_c-act and T_p of corn and E under DI and SDI during the corn growing season are shown in Fig. 3. The ET_c-act varied from 2.1 to 9.7 mm/day, depending on the crop growth stage and climatic condition. The daily ET_c-act and T_p increased rapidly and peaked 48 days after planting at the mid-season stage with average values of 9.7 and 7.6 mm/day under DI and 9.5 and 8.7 mm/day under SDI. The minimum ET_c-act values occurred at the initial stage with an average of 2.5 and 2.1 mm/day under DI and SDI. During the first week after planting, the most share of ET_c-act was E, because the soil was kept wet for corn seed germination. Transpiration increased faster un-der the SDI than the DI (Fig. 3) because of moisture distri-bution close to the root mass and better root development under the SDI at the initial stage.

Significantly lower corn ET_c-act and E values were ob-tained under the SDI than under DI but for both seasons, the T_p was higher under SDI than DI (Table 2). The avera-ge annual ET_c-act was 423.49 mm under DI and 384.84 mm under SDI. Similar results have been reported by Chuan-yan & Zhongren (2007), Liu et al. (2017) and Zhang et al. (2017). Evaporation accounted for a small proportion of ET_c-act and decreased with time and increasing LAI. Trans-piration was smaller than E at the initial growth stage (Fig. 3), and the total contribution to ET_c-act at that stage was from E because more water was lost through soil surface evaporation and the crop canopy was not fully developed yet as reported in Allen et al. (1998) and Valentin et al. (2020). The T_p increased to a peak at the mid-season stage (Liu et al., 2002; Kang et al., 2003; Majnooni-Heris et al., 2012). During this stage, E was reduced and the reduction was compensated with higher amounts of water for T_p, coinciding with increasing LAI until the crop achieved near or full ground cover (Table 2).

The average annual T_p during the two growing seasons was 221.9 and 250.2 mm, and that for E was 198.0 and 157.6 mm under DI and SDI, respectively. Under SDI, the soil surface usually remained drier than under DI. Ac-cordingly, the total T_p was 53.0 and 64.2% of corn ET_c-act under DI and SDI. Also, the average annual E was 46.9 and 35.9 % of corn ET_c-act under DI and SDI during two growing seasons (Table 2). At the initial stage during which the portion of the bare or low covered soil surface was high and considering that under DI, moisture accumu-lation occured on the soil surface, the DI showed 27.70% more evaporation (82.9 mm) than SDI (64.9 mm).

 

Relationship between the ratio of plant transpi-ration (T_p) and soil evaporation (E) to actual corn evapotranspiration (ET_c-act) and leaf area index (LAI)

Based on the soil-plant-atmosphere continuum (SPAC), the variations in T_p, and ET_c-act are affected by meteorological data, soil moisture and plant factors (Zhou et al., 2017). The variations in T_p/ET_c-act and LAI for each growing season are presented in Fig. 4. The data indica-ted that the ratio was controlled by soil surface cover and LAI. The average T_p/ET_c-act varied from 0.0 at sowing to 82 and 87% at full growth under DI and SDI. The value of T_p/ET_c-act increased faster when LAI was smaller than 3.0. Thus, two polynomial functions were computed to show the relation between the ratio of T_p/ET_c-act and E/ET_c-act with LAI under SDI and DI (Eqs. 8 and 9; Fig. 4), as reported by other authors (e.g., Kang et al., 2003; Maj-nooni-Heris et al., 2012).

 

It was presumed, as occurred for the T_p/ET_c-act ratio, that the E/ET_c-act was also affected by LAI and surface soil moisture (Liu et al., 2002). Fig. 5 shows the trends in the ratio of E/ET_c-act with LAI under DI and SDI during the growing seasons. The E/ET_c-act decreased significantly witth increases in LAI. The relationship between LAI and E/ETc-act based on the experimental data was calculated as follows:

 

From these relationships, the E/ET_c-act decrea-sed sharply when LAI was at about 3, which was attri-buted to lower evaporation due to the development of crop canopy (Fig. 5). Under SDI, the soil surface be-tween crops usually remains dry, so that evaporation loss from the soil surface is low but moisture distribution at the root zone is suitable for crop growth. Thus, the SDI can improve water uptake by reducing soil evaporation (Liu et al., 2002).

 

Variations in basal crop coefficient (K_cb), soil eva-poration (K_e) and LAI

 

The mean variations for the two years for K_cb-m and K_e-m compared to K_cb-adj, and K_e-adj are presented in Fig. 6. The values of K_cb-m increased from 0.0 to its peak value in the mid-season stage. The average, minimum and maximum values of K_cb-m were 0.47, 0.0 and 1.52 under DI and 0.53, 0.0 and 1.74 under SDI; for K_e-m the values were 0.33, 0.20 and 0.58 under DI and 0.23, 0.15 and 0.46 under SDI, respectively. The peak value of K_cb-m was observed in the mid-season stage when LAI was maximum.

For the whole growing season, the values of K_cb-m were smaller than the adjusted values based on FAO-56. This was attributed to the overestimation of ET_o by FAO-Penman-Monteith model for this region (Dehghani-sanij et al., 2004).

For the different irrigation systems, the peak values of K_cb-.m were obtained under SDI at the mid-season stage, and the K_e-m under the DI peaked at the initial stage. The K_e-m decreased with crop development during the growing season due to the increase in the percentage of the shaded area by the plant canopy. Again, Fig. 6 emphasizes that the evaporation from soil surface was higher than the transpiration from the crop at the initial stage and with an increase in plant shading, evaporation was lower than transpiration during crop development and mid-season stages. Moreover, the K_e-m varied temporally during thecorn growing season. Thus, the average K_e-m peaked at the initial stage, and decreased gradually during the growing season, reaching a minimum value at the mid-season stage (Table 3).

The amounts of K_cb-FAO during the initial stage, crop development and mid-season stage of corn growth (K_cbini, K_cb-dev, and K_cb-mid) were 0.15, 0.15-1.15 and 1.15, respectively (Allen et al., 1998). The values of K_cb-dev and K_cb-mid changed on the basis of plant height, wind speed and relative humidity in different regions. Therefore, the recommended K_cb values were adjusted to 0.15, 0.70, and 1.20 under DI and 0.15, 0.76, and 1.29 under SDI during the initial stage, crop development, and mid-season stages, respectively (Table 3). The maximum value of K_cb-adj was 1.30 under SDI at the mid-season stage, which was attributed to the higher Tp. The K_e-adj varied temporally during the corn growing season. Also, the K_e-adj value was higher at the initial stage and gradually decreased, reaching a minimum value of only 0.14 under SDI at the mid-season stage (Fig. 6).

The K_cb-adj and K_e-adj were higher than K_cb-m and K_e-m during the initial stage and crop development. Whe ther for DI or SDI, the K_cb-adj values were overestimated when compared to K_cb-m for the whole growing season. Comparisons of K_cb-m and K_e-m vs K_cb-adj, and K_e-adj values are shown in Fig. 7. For both DI and SDI, the relationships between K_cb-m and K_e-m as well as K_cb-adj, and K_{e -adj} were linear linear. The K_{cb -adj} under DI performed better than that under SDI. The slopes of the linear regression were 0.93 and 0.85 with a coefficients of determination of 0.75 and 0.71 for K_{cb -adj} under DI and SDI, respectively.

The K_e-adj under DI and SDI was overestimated as compared to K_e-m during the initial and crop development stages and underestimated during the mid-season stage. Also, K_e-adj under DI provided a better performance than that under SDI. The slopes of the linear regression were 0.67 and 0.67 with a coefficients of determination of 0.25 and 0.19 for K_{e -adj} under DI and SDI, respectively.

The difference between K_cb-m and K_e-m and adjusted values by FAO-56 clearly emphasizes the difficulty of applying K_cb and K_e values across locations due to varying climatic and agricultural management factors like irrigation method and frequency (Katerji & Rana, 2014).

As a response to the crop development, the K_e was higher than the K_cb at the initial stage, and with an in-crease in LAI and plant shading, the resulting de-creases in soil surface evaporation could be used to estimate the variations in K_cb and K_e as functions of LAI (Fig. 8).

Overall, the relationship between LAI and K_cb and K_e based on the experimental data was calculated using se-cond-order polynomial equations with high coefficients of determination (R²) as follows:

The K_cb of corn at the mid-season stage was larger be-cause of the large LAI at this stage and consequently, the smaller soil evaporation, compared to plant transpiration.

Similar relationships have been reported for bean and canola (De Medeiros et al., 2001; Majnooni-Heris et al., 2012).

 

 

Table 2. The ratio of soil evaporation (E) and plant transpiration (T_p) to actual corn evapotranspiration (ET_c-act) under surface drip irrigation (DI) and subsurface drip irrigation (SDI) systems during 2014 and 2015 corn growing seasons.

 

 

 

Variations in leaf area index (LAI), plant height (H), stem diameter (D), wet and dry mass yield and water use efficiency (WUE)

The variations in mean LAI, H, D, biomass yield and WUE under DI and SDI are presented in Table 4 and Fig. 9. The LAI increased slowly in the initial stages and more rapidly reached its peak at about 3.4 m²/m² under DI and 4.6 m²/m² under SDI. Also, the H increased slowly at the initial stages and more rapidly at the mid-season stage, with the tallest plants (229.2 cm) under SDI at the mid-season stage. Moreover, the D also increased during the growing season and peaked during mid-season with values of 3.2 cm under DI and 3.6 cm under SDI. Between the irrigation systems, the lowest values of LAI, H and D occurred under DI (Table 4 and Fig. 9) as reported in Chuanyan & Zhongren (2007).

The highest yield (biomass) was 81.90 ton/ha under SDI compared to only 63.21 ton/ha under DI. Also, the WUE (kg/m³) of corn was 8.32 under SDI and 6.67 under DI (Table 4). This better performance under SDI could be explained by the conservation of optimal moisture status in the root zone, which favoured water and nutrient up-take by the crop (Zotarelli et al., 2008; Badr et al., 2010).

In summary, the results of this study showed that the lowest ET_c-act for corn occurred under SDI, mainly because of reduced evaporation (E) from the soil surface in comparison to the DI system. Consequently, higher T_p and lower E rates were observed under the SDI system. The T_p/ET_c-act started from 0 at sowing and peaked at the mid-season stage when the LAI was also at peak levels. The DI and SDI systems had different influences on the LAI of corn during the growing season. A better unders-tanding of these components (T_p, E, E/ET_c-act and T_p/ET_c-act) can provide important insights to water saving under irrigated corn production. The highest LAI occurred un-der the SDI system. Overall, the SDI system reduced soil evaporation loss and increased the efficiency of water consumption; it also produced superior biomass yield. Thus, the SDI system is more productive and would better increase WUE than the DI system.

 

Table 3. Mean values of measured (m) and adjusted (adj) soil evaporation coefficient (K_e) and basal crop coefficient (K_cb) under surface drip irrigation (DI) and subsurface drip irrigation (SDI) systems during 2014 and 2015 corn growing seasons.

 

 

Table 4. Corn yield and growth attributes under surface drip irrigation (DI) and subsurface drip irrigation (SDI) during 2014 and 2015 corn growing seasons.

AcknowledgmentsTop

The authors wish to render heartfelt gratitude to Egrin-ya Eneji, Professor of Agronomy & Honourable Commis-sioner, Ministry of Training and Doctrine, Cross River State, Nigeria, for carefully editing the manuscript lin-guistically and technically.