Partitioning evapotranspiration into soil evaporation and transpiration using a modified dual crop coefficient model in irrigated maize field with ground-mulching

doi:10.1016/j.agwat.2013.05.018

Agricultural Water Management

Volume 127, September 2013, Pages 85-96

https://doi.org/10.1016/j.agwat.2013.05.018 Get rights and content

Highlights

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We reported a modified dual crop coefficient (K_c) model to partition evapotranspiration.
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Daily basal K_c was dynamically calculated by introducing canopy cover coefficient.
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Leaf senescence factor was taken into consideration to modify basal K_c.
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Evaporation coefficient was modified by accounting for effect of ground-mulching.
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The good agreements were found through comparing measurements with predictions.

Abstract

The accurate partitioning of crop evapotranspiration (ET_c) into two components, soil evaporation (E_s) and transpiration (T_r), is needed to better understand terrestrial hydrological cycles and develop precise irrigation scheduling. However, there is no easy way to distinguish between the two. Based on FAO-56 dual crop coefficient (K_c) approach, we developed a modified dual K_c model for better predicting T_r through basal crop coefficient (K_cb) and E_s through evaporation coefficient (K_e). Daily K_cb was dynamically calculated by introducing a canopy cover coefficient that could be simply described as a function of leaf area index or fraction of canopy cover. Also, leaf senescence factor was taken into consideration to modify K_cb when leaf suffers functional senescence. K_e was modified through introducing the fraction of ground-mulching (f_m) to account for the effect of mulching on E_s. The model was parameterized by measurements in 2009, and validated using independent data for grain and seed maize with and without mulching in 2010 and 2011. The results indicate that the predicted K_c values by the modified model were obviously better than those by the original model. The good agreements were found between the predicted ET_c, T_r and E_s using the modified model and the measurements for grain maize in 2010 with f_m = 0.6, with the slope of linear regression of 0.99 (R² = 0.90), 1.01 (R² = 0.92) and 0.96 (R² = 0.78), respectively. The modified model also well reproduced the values of ET_c and E_s for seed maize in 2011, which had lower plant height and leaf area index compared to grain maize, under mulching (f_m = 0.7) and non-mulching (f_m = 0) conditions. The slopes of linear regression between predictions and measurements were 0.98 (R² = 0.91) and 0.99 (R² = 0.92) for ET_c, and 0.98 (R² = 0.79) and 0.97 (R² = 0.80) for E_s under f_m = 0.7 and 0.0, respectively. These results suggest that the modified dual K_c model can accurately predict ET_c, E_s and T_r for different crop types under different mulching, thus could be a useful tool for improving irrigation water management.

Introduction

Crop evapotranspiration (ET_c) plays a key role in energy and water balance of agricultural systems, and is also a key process in the terrestrial hydrological cycle (Burba and Verma, 2005, Zhang et al., 2011a). More than 90% of water used in agriculture is lost by soil evaporation (E_s) and crop transpiration (T_r), referring to as ET_c (Rana and Katerji, 2000). Transpiration is strongly linked to crop productivity since it occurs simultaneously with photosynthesis through the stomatal pores of leaves (Pieruschka et al., 2010). Conversely, direct soil evaporation is not a contributing factor to crop production, and should be reduced by management practices (e.g., proper irrigation strategies and ground-mulching) (Allen, 2000, Zhao et al., 2010). Whereas the two separate processes of T_r and E_s occur simultaneously, and there is no easy way to distinguish between the two (Er-Raki et al., 2010). Therefore, accurate partitioning between soil evaporation and transpiration by models is needed to better understand terrestrial hydrological cycles, develop precise irrigation scheduling, improve crop productivity and enhance water use efficiency.

Several models have been developed to separately predict soil evaporation and transpiration since direct measurement of them is difficult, costly and not available in many regions (Allen et al., 1998, Monteith and Unsworth, 2008, Shuttleworth, 2007, Utset et al., 2004). The direct models include both the single-layer Penman–Monteith and two-layer Shuttleworth–Wallace (Monteith and Unsworth, 2008, Shuttleworth and Wallace, 1985). Many input parameters of these models cannot be easily obtained, thus wide application of these models is limited (Allen, 2000). In addition, weather data are routinely measured only above a grassed surface, so the application of the direct models is limited since the differences may exist between the characteristics of the weather measurements and the feedback that should be presented to reflect ET_c of the surface in question (Jarvis and McNaughton, 1986). In contrast, the indirect FAO-56 crop coefficient (K_c) approach has overcome these deficiencies since K_c has contained all differences between the reference surface and the physiologies, physics and morphologies of crop in question (Allen, 2000, Allen et al., 1998). Especially, the dual K_c model has been a preferred approach due to its simplicity for fewer input data and robustness for separately predicting E_s and T_r (Allen, 2000, Er-Raki et al., 2010). The dual K_c model has been widely used in scheduling irrigation and improving agricultural production (Allen, 2000, Liu and Luo, 2010, Paço et al., 2011, Zhao and Ji, 2010).

The basal crop coefficient, K_cb, is generally obtained from the guideline of FAO-56 by looking-up the tabulated value at every growth stage and then linearly interpolated to obtain daily values. The approach from the original FAO-56 dual K_c procedures cannot calculate daily actual value of K_cb, although daily actual value is of great importance for accurately calculating the dynamic of T_r. Furthermore, leaf senescence and the decline of physiological function at the late season stage could induce stomatal closure and sharp decrease of T_r (Ding et al., 2013, Steduto and Hsiao, 1998). The original dual K_c model has not taken into consideration the effect of leaf senescence. Thus, it is essential to modify the original dual K_c approach to accomplish daily dynamic calculation of K_cb. Maize, including grain and seed maize, is one of the main crops in arid regions of northwest China, and its water requirement is mainly supplied by irrigation because of low precipitation (Ding et al., 2010, Zhao et al., 2010). To reduce E_s, ground is often mulched with plastic film, which is a well-established management strategy and widely used (Ding et al., 2013, Hou et al., 2010, Zhou et al., 2009). Whereas evaporation coefficient (K_e) in FAO-56 was obtained by available energy and soil moisture regimes at the topsoil layer, the effect of ground-mulching on E_s was not accounted in K_e. As we have known, E_s would definitely decrease when ground is mulched, because the evaporation area is reduced by ground-mulching. The previous studies have indicated that E_s was reduced by ∼50% with plastic film mulching over the whole growing season, especially during the early growing stages where soil surface was not fully covered by crop canopy (Hou et al., 2010, Mukherjee et al., 2010, Zhou et al., 2009). At present, an analytical formula of K_e incorporating the effect of mulching on E_s in dual K_c model has not been established yet.

In this study, to better partition evapotranspiration into soil evaporation and crop transpiration, a modified dual K_c model was developed through accomplishing daily dynamic estimation of K_cb, and incorporating the effects of leaf senescence and ground-mulching on T_r and E_s. Its performance was tested through comparing the predicted ET_c, T_r and E_s with measurements in irrigated grain and seed maize with and without mulching in northwest China.

Section snippets

The modified dual crop coefficient model

In FAO-56, the actual ET_c is defined as the product of crop coefficient (K_c) and reference evapotranspiration (ET_o) (Allen et al., 1998). $E T_{c} = K_{c} E T_{o}$

In the dual K_c model, K_c is split into two factors that separately describe the evaporation (K_e) and transpiration (K_cb) components. $K_{c} = K_{s} K_{c b} + K_{e}$ where K_s is water stress coefficient whose value is dependent on available soil water in the root zone. The corresponding crop transpiration (T_r) and soil evaporation (E_s) are calculated as follows: $T_{r} = K_{s} K_{c b} E T_{o}$ $E_{s}$

Study area and experimental arrangement

The experiments were conducted at Shiyanghe Experimental Station for Water-saving in Agriculture and Ecology of China Agricultural University, located in Gansu Province of northwest China (N 37°52′, E 102°50′, altitude 1581 m) during 2009–2010. The site has long sunlight hours with a mean annual sunshine duration over 3000 h, mean annual temperature of 8 °C and frost-free days of 150 d. The region is limited in water resources with a mean annual precipitation of 164 mm and a mean annual pan

Model parameterization and calibration

The water depletion fractions for non-stress conditions (p) were obtained through inverse methods based on 2009 data (Table 2). The initial p value was set as 0.5 during the whole growing period. The iteration terminal criterion was set as less than 5% differences between the measured and predicted K_c averages at every growing stage. The p decreased from 0.55 at the initial stage to 0.45 at the mid-season stage (Table 2). The higher value of p at the initial stage was attributed to lower

Conclusions

The original FAO-56 dual crop coefficient model was modified through introducing a canopy cover coefficient (K_cc), leaf senescence factor (f_s), and fraction of ground-mulching (f_m). The dynamical basal crop coefficient (K_cb) for predicting transpiration (T_r) was calculated using K_cc. Also, the effect of f_s on reduction of T_r was incorporated in K_cb. After accounting for the effect of f_m on soil evaporation (E_s), evaporation coefficient (K_e) for predicting E_s was modified. Observed crop

Acknowledgments

We are grateful to the research grants from the National High-Tech Research and Development Program (2011AA100502, 2013AA102904, and 2013AA103004), the Chinese National Natural Science Fund (51222905 and 91225301), and the Special Fund for Water-scientific Research in the Public Interest, Ministry of Water Resources (201201003). The research is also supported by Chinese Universities Scientific Fund (2013XJ018 and 2013QJ042). The authors also acknowledge the two anonymous reviewers for their

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Partitioning evapotranspiration into soil evaporation and transpiration using a modified dual crop coefficient model in irrigated maize field with ground-mulching

Highlights

Abstract

Introduction

Section snippets

The modified dual crop coefficient model

Study area and experimental arrangement

Model parameterization and calibration

Conclusions

Acknowledgments

Journal of Hydrology

Agricultural and Forest Meteorology

Agricultural and Forest Meteorology

Agricultural Water Management

Agricultural Water Management

Agricultural and Forest Meteorology

Advances in Ecological Research

Agricultural Water Management

Agricultural and Forest Meteorology

Agricultural Water Management

Agricultural Water Management

European Journal of Agronomy

Agricultural and Forest Meteorology

Agricultural and Forest Meteorology

Agricultural Water Management

Agricultural Water Management