1. Introduction
Crop evapotranspiration (ET
c) is one of the key indicators of field water management, crop irrigation scheduling, and planning and design of farmland water conservancy projects [
1]. ET
c is divided into two parts, soil evaporation (E
s) and plant transpiration (T
r). Among them, E
s, known as ineffective water consumption for crop growth and yield, can be decreased by ground coverage or proper irrigation management [
2,
3]. T
r, associated with photosynthetic carbon fixation through leaf pores, directly decides crop growth and the final yield [
4]. However, as two water consumption processes in the farmland, T
r and E
s occur simultaneously, so it is difficult to carry out quantitative partitioning. Therefore, accurate determination of crop evapotranspiration and its components is of great significance for guiding field irrigation and improving the water use efficiency.
The FAO-56 dual crop coefficient approach is widely used because it can be used to accurately estimate crop evapotranspiration and realize quantitative partitioning of daily E
s and T
r [
5]. Fan and Cai [
6] and Lu et al. [
7] demonstrated that ET
c can be accurately estimated by the dual crop coefficient approach. A micro-lysimeter can be used to measure E
s, but owing to the limited measuring accuracy of the instrument, the accuracy can merely be controlled within 15–20% [
8]. Rosa et al. [
9] developed a dual crop coefficient model (SIMDualK
c) based on the dual crop coefficient approach, making it easier to partition ET
c. Many studies showed that the model has a highly accurate estimation of ET
c and its components for wheat, maize, forage, tomato, chili, pea, cucumber, etc., in Brazil, Uruguay, Portugal, Spain, and North China [
10,
11,
12,
13,
14,
15,
16,
17].
Agricultural irrigation is a large water user in arid regions of Northwest China, which is short of water resources, so the use of a new effective water-saving irrigation technology is of great strategic significance for ensuring the water resources security and ecological safety of Northwest China [
18]. Drip irrigation under mulch, which is a new type of water-saving technology integrating the advantages of the mulch film, such as soil temperature conservation, soil moisture conservation, yield increase, and the water-saving advantage of drip irrigation, can be used to decrease E
s and increase water use efficiency utilization during the initial stage of crop growth [
19]. Thus, it has been widely used in arid regions of Northwest China. Previous studies have indicated that E
s was reduced by ~50% with plastic film mulch over the whole growing season [
20,
21,
22]. Fan et al. [
23] indicated that plastic mulch decreases the available energy and ET
c of maize in an arid region of northwest China, and thus the crop coefficient (K
c). Ding et al. [
20] introduced a ground-mulching factor to modify the original soil evaporation coefficient in order to account for the reduction of the evaporation area by plastic film mulch. Zhang et al. [
24] found that maize ET
c with drip irrigation under mulch was reduced by 2.8–5.2%, with reduced soil evaporation by 45.2% and increased transpiration by 8.9% in Northeastern China. However, there remain very few studies on ET
c and its components related to the use of drip irrigation under mulch in arid regions of Northwest China.
Crop regulated deficit irrigation (RDI) is a water-saving and high-yield irrigation technology based on the relationship between crops and water. Moderate water deficit in the growth stage of crops can reduce crop water consumption but has a small impact on the final grain yield, thereby improving water use efficiency [
25]. RDI reduces crop water consumption mainly by reducing crop growth and leaf area or canopy coverage, but a reduction in canopy coverage will increase the area of bare soil and increase soil evaporation. For example, water deficit in the seedling or early growth period would delay crop growth and canopy cover time, increasing the proportion of ineffective soil evaporation [
26]. After the canopy covers the ground (or the leaf area index is greater than 3.0 m
2 m
−2), the implementation of the strategic stage of deficit adjustment can ensure the reduction of crop water consumption without increasing the proportion of soil evaporation [
27]. Therefore, the timing of RDI is very important to reduce crop water consumption without increasing ineffective soil evaporation.
In this study, a two-year field experiment of maize with drip irrigation under mulch was carried out, and two water treatments were set up, namely full irrigation (T1) and strategic stage regulated deficit irrigation in the late growth and reproductive periods (T2). The SIMDualKc model was used to estimate the ETc and Es and Tr of maize during the whole growth period. The objectives were: (1) to quantify the proportion of ETc and Es and Tr of maize with drip irrigation under mulch, and (2) to compare the differences in water use between the two treatments. These results provide a novel approach for efficient water management by strategic growth stage-based RDI in field maize.
3. Results and Discussion
Table 3 shows the initial and calibration values of the main model parameters. After calibration, the K
cb of maize with drip irrigation under mulch at the initial stage (K
cb-ini), mid-season stage (K
cb-mid), and late season stage (K
cb-end) were equal to 0.2, 1.15, and 0.55, respectively. The values of K
cb obtained in this study were similar to those in the existing studies and sit within the reviewed and updated range of K
cb for field maize based on accurate crop ETc measurement and FAO56 method by Pereira et al. [
34]. Chauhdary et al. [
35] presented K
cb-mid = 0.93, K
cb-end = 0.47 for dripped maize with high grain moisture; they used the SALTMED model and gravimetric SWC measurements in Faisalabad, Pakistan. The experimental results achieved by Gimenez et al. [
11] in western Uruguay showed that K
cb-ini = 0.15, K
cb-mid = 1.05, and K
cb-end = 0.3. Martins et al. [
36] studied maize with sprinkling irrigation and drip irrigation under organic film in southern Brazil and showed that K
cb-ini = 0.2, K
cb-mid = 1.12, and K
cb-end = 0.2. Rodrigues et al. [
37] conducted a study on maize under full irrigation and deficit drip irrigation in Portugal and found that K
cb-ini = 0.15, K
cb-mid = 1.15, and K
cb-end = 0.4. Paredes et al. [
38], in Portugal, showed by using the AquaCrop model that K
cTr,x = 1.18. Paredes et al. [
12] in 2014 showed that K
cb-ini = 0.15, K
cb-mid = 1.15, and K
cb-end = 0.3. Yan et al. [
39] studied summer maize under different drip irrigation conditions using the SIMDualK
c model in Yangling, Shaanxi, concluding that K
cb-ini = 0.15, K
cb-mid = 1.13, and K
cb-end = 0.2. Zhao et al. [
40] studied summer maize in Beijing, concluding that K
cb-ini = 0.2, K
cb-mid = 1.1, and K
cb-end = 0.45. Li et al. [
25] studied maize by drip irrigation under mulch in northeastern Inner Mongolia, concluding that K
cb-ini = 0.15, K
cb-mid = 1.05, and K
cb-end = 0.4. The slightly higher K
cb-end might be due to the incomplete senescence of maize.
The measured and simulated SWC in the root zone of the two treatments in 2017 and 2018 are shown in
Figure 3. The goodness-of-fit statistic of calibration and verification are shown in
Table 4. The simulated value and measured SWC fit well. The simulated SWC can capture a dynamic process in which the SWC increased in a short period with irrigation or rainfall, and then gradually decreased due to ET
c. The regression coefficient b was 0.96–1.07, R
2 was 0.84–0.95, RMSE was 0.005–0.008 cm
3 cm
−3, AAE 0.01 was cm
3 cm
−3, E
max 0.025 was cm
3 cm
−3, and d
IA reached up to 0.96, which was better than the results of the study of rain-fed maize in Inner Mongolia by Wu et al. [
41]. These results were slightly lower than those found by Zhao et al. [
39] on summer maize in Beijing (b = 0.91–1.01, R
2 = 0.87–0.93), but the relative error of SWC in this study was lower than 10%, suggesting that the SIMDualK
c model was accurately able to calculate SWC and can be used to calculate ET
c of maize and its partitioning [
9].
The E
s, T
r, and ET
c of maize were estimated using the calibrated and verified SIMDualK
c model. Daily K
e, K
cb, and K
cbadj, as well as E
s, T
r, and ET
c, and measured T
r for T1 and T2 in 2017 and 2018 are shown in
Figure 4 and
Figure 5, respectively. The goodness-of-fit statistics of the measured and simulated T
r are presented in
Table 5. The simulated and measured T
r had the same changing trend during the growth period. The b was 0.91–1.04, R
2 was 0.91–0.97, RMSE was 0.366–0.389 mm d
−1, AAE < 0.5 mm d
−1, E
max was 1.163 mm d
−1, d
IA > 0.95, and EF 0.80–0.91. Although T
r was only verified during the mid-to-late growth period, we concluded that the model can estimate T
r throughout the growth period since it accurately simulated SWC throughout the growth period. Qiu et al. [
42] compared tomato ET
c measured by a lysimeter with SIMDualK
c simulations and found that b was 0.91–1.13 and R
2 was 0.55–0.82. Yan et al. [
17] compared measured T
r values of greenhouse cucumber with simulations and demonstrated that the R
2 was 0.89–0.92 and RMSE was 0.36–0.51 mm d
−1. Our results were similar to those of previous studies. Overall, after being calibrated, the SIMDualK
c model can better simulate the changes in ET
c of maize with drip irrigation under mulch during the growth period.
E
s and T
r values and their ratios to ET
c in different growth stages of maize are shown in
Table 6. In 2017, the ET
c for T1 and T2 was 507.9 and 428.9 mm, E
s was 32.0 and 43.6 mm, and T
r was 476.0 and 385.3 mm, respectively during the whole growth period of maize. In 2018, the ET
c for T1 and T2 was 519.1 and 430.9 mm, E
s was 35.2 and 43.4 mm, and T
r was 484.0 and 387.5 mm, respectively during the whole growth period of maize. There were large differences in ET
c, E
s, and T
r between T1 and T2. In particular, there was a difference of 90.7–96.5 mm in T
r, which occurred in the middle growth period. The pattern was similar for two years, which suggests that drip irrigation with film mulching can significantly reduce soil evaporation regardless of whether full or regulated deficit irrigation are used.
Tr was the major component of ETc, with the Tr/ETc ratio of 93.7% and 89.8% for T1 and T2 in 2017, and 93.2% and 89.9% in 2018, respectively. Although Tr and ETc decreased for T2, the Tr/ETc ratio did not decrease significantly, suggesting that the growth-based RDI strategy maintains a higher percentage of crop effective transpiration. The Es/ETc ratio obtained for T1 in the two years was 6.3% and 6.8%, while it was 10.1% and 10.2% for T2, respectively. T2 caused higher evaporation than T1 for the reason that T2 restricted the growth of maize and the fc for T2 was lower than that for T1 at the mid-season stage and late-season stage, causing an increase in the exposed soil area, thus increasing the Es. In the early stage of growth, the fc of maize was very low, with Es as the major active component, and the Es/ETc ratio was highest, in the range of 39–49.4%. At the development stage, the evaporation ratio was 9.9–12.2% in 2017 and 1–1.6% in 2018. Such a large difference was due to a decrease of 18.4 mm in rainfall and a decrease in irrigation volume of 7.2 mm in the same period in 2018. The Es in the mid-growth period in 2017 was smaller than that in the late-growth period, and the opposite was true in 2018. This was because the rainfall in the mid-growth period in 2018 was 140.2 mm, which was 57.6 mm more than in 2017, which led to an increase in soil evaporation. These results indicated that soil evaporation is greatly affected by the degree and coefficient of soil surface moisture and canopy coverage. For efficient crop water management practices, inefficient water consumption can be minimized by covering the ground with the canopy as soon as possible before performing deficit irrigation.
Previous studies have shown that drip irrigation under mulch can effectively reduce soil evaporation, thus improving the effective water use efficiency of crops or increasing T
r/ET
c, thereby promoting the growth of biomass and yield [
23]. Ding et al. [
20] found that for maize for seed under film conditions (f
m = 0.7) in arid regions of Northwest China, E
s decreased by 55.7% compared to film-free conditions, while T
r was higher. Martins et al. [
36] found that the E
s/ET
c ratio in a maize field was 8–9% under drip irrigation with straw mulch. Li et al. [
19] found that that the maize E
s/ET
c ratio was 19.85–20.29% with film-mulched treatment but 26.15–27.23% without mulch in northeastern Inner Mongolia. Kang et al. [
43] studied irrigated maize without mulch in the Guanzhong area, concluding that the E
s/ET
c ratio was 26%. In this study, the E
s/ET
c ratio of the two treatments under the condition of mulching drip irrigation were 6.3–10.2%, which is lower than the results of previous studies, indicating that drip irrigation under mulching mainly increases the effective transpiration rate of crops by reducing soil evaporation to save water and increase yield.
The E
s/ET
c ratios were 10.1% and 10.2% for T2 for the two years, respectively, which is slightly higher than those of T1, at 6.3–6.8%. We started to implement water deficits in the late growth period after the canopy covered the ground, which might cause leaf curling, reduce the canopy coverage, and increase the area of bare soil and evaporated surface. Comas et al. also found that in addition to reducing crop growth and leaf area, water deficit also increased the proportion of rolled leaves, thereby reducing canopy coverage [
27]. In this study, due to the use of drip irrigation under the mulch, the area of irrigated wetness and bare soil was small. Even though RDI reduced the canopy coverage and increased the bare soil area, the actual wet soil evaporation area did not increase, so there was no significant increase in E
s. These results indicate that in the practices of efficient water management for crops, sufficient irrigation in the early stage of growth can be used to quickly cover the ground in the canopy and then implement the strategic stage of RDI. At the same time, combined with high-efficiency water-saving irrigation methods such as drip irrigation under mulch, it can reduce water use but does not increase the proportion of effectless water.
Although our study area is arid and cold with an annual average temperature of 8 °C, our methods and result patterns can be extended to other areas. The purpose of our study was to estimate ET
c and its components to support irrigation scheduling using the SIMDualKc model based on daily soil water balance. The estimation accuracy can be improved if ones take into account soil water infiltration together with the root water uptake [
44,
45,
46]. Further work will be needed to incorporate the two processes into dynamic soil water equations, e.g., using the Richards equation, for accurate partitioning of ET
c and soil water flow.