Irrigation water requirements for seed corn and coffee under potential climate change scenarios

Population growth, climate change, and anthropogenic land-use changes trigger severe negative effects on the global water resources. The impacts of these changes have more pronounced effects on small elevation-variant islands, e.g., Hawaiian Islands. These islands have been experiencing prevailing spatio-temporal variability of the major water cycle components, e.g., rainfall, evapotranspiration, and to a less extent temperature. Human activities are altering the global climate (Hansen et al. 2006) by raising the concentration of the greenhouse gases especially methane, nitrous oxide and carbon dioxide (CO₂). Atmospheric CO₂ is expected to increase from its reference (330 ppm) concentration used in this study to between 550 and 970 ppm by 2,100 (IPCC 2007). Increased greenhouse gas concentrations are likely to raise the earth's mean temperature and influence precipitation, storm patterns and sea level (IPCC 2007). However, in general, the magnitude of these changes will depend on future anthropogenic activities as well as on technological and economic development. Over the past years of the 21st century, the global average surface temperature has increased by approximately 0.6 °C and is projected to rise by an additional 1.1–6.4 °C (IPCC 2007).

Timm & Diaz (2008) analyzed the precipitation variations over the Hawaiian Islands during most of the first decade of the 21st century using the AR4 A1B emission scenario. They predicted a 5–10% reduction of wet season precipitation and 5% increase during the dry season. Karl et al. (1996) reported an increase of the mean air temperature by 2.5 °C and 20% decrease in precipitation between 1,900 and 1,990 based on analysis of observed air temperature and precipitation at Honolulu International Airport, Hawai'i. Giambelluca & Luke (2007) reported an increasing trend in the mean annual temperature between 0.12 and 0.23 °C per decade across the Hawaiian Islands between 1905 and 2006. Although the magnitude of change in temperature and precipitation varied temporally and spatially, the anticipated impact on the hydrology and thus, irrigation water requirements for the crops in the mountainous Hawaiian watersheds is a serious matter for sustainable future water yields.

Anticipating changes in the hydrologic cycle is particularly important for regions with limited water supplies (Ficklin et al. 2009) such as the Hawaiian Islands. The Hawaiian Islands are the most isolated oceanic island group on earth and rely solely on precipitation and subsequent groundwater recharge for their fresh water needs (Mair & Fares 2010). Streams in Hawai'i supply more than 50% of irrigation water and a fair amount of drinking water in some places (Oki 2003). Examining the sensitivity of hydrologic responses to climate across different geomorphologic regions is important to formulate appropriate water management policies and irrigation water allocations for local responses (Qi et al. 2009).

An increase in average daily temperature due to climate change will increase reference evapotranspiration (ET_o) resulting in an increased use of irrigation water whereas increase in atmospheric CO₂ will increase crop water use efficiency (WUE, which is the ratio of CO₂ uptake to evapotranspiration) resulting in less water needed for irrigation (Ficklin et al. 2010). The likely net effect of increased temperature and CO₂ will be insignificant changes in ET_o within the next 30 years (Hatfield et al. 2008).

Elevated atmospheric CO₂ often reduces plant stomatal conductance (Nederhoff et al. 1992; Rodaglou et al. 1992) and plant transpiration per unit leaf area (Goudriaan & Unsworth 1990; Dugas et al. 1997). Reduced transpiration increases the leaf temperature which can further increase photosynthesis (Acock 1990). The effect of higher levels of CO₂ on stomatal conductance and transpiration is observed in both C3 and C4 species (Dugas et al. 1997). Morison & Gifford (1984) found that stomatal conductance was reduced by 36% while transpiration was reduced by 21% for several plant types, the difference being attributed to the higher leaf temperatures. Similar average values of 34% for stomatal conductance and 23% for transpiration were found in a literature survey by Cure & Acock (1986). An increase in photosynthesis and a decrease in transpiration result in an increase in the plant's water use efficiency.

Many studies estimated future changes in irrigation water requirements without considering a decrease in stomatal conductance due to elevated CO₂ in calculating ET_o. These studies calculated ET_o using temperature-based equations, e.g. Modified Hamon equation (Lee & Huang 2014), Priestley-Taylor method (Doll 2002). Several investigators, e.g., Strzepek et al. (1999), Hatch et al. (1999), and Rosenzweig & Iglesias (1998), used the Priestley-Taylor method to calculate ET_o; in addition, they incorporated the effect of elevated CO₂ on stomatal closure and leaf area index and consequently, their impact on potential transpiration using the ratio (Peart et al. 1989) of transpiration under elevated CO₂ conditions to that under ambient conditions. However, very few researchers have attempted to estimate future changes in irrigation water requirements based on projected climate changes from GCMs, and estimates of decreased stomatal conductance due to elevated CO₂ in ET_o (e.g., Allen et al. 1991; Izaurralde et al. 2003). Thus, in this study we used a modified version of the FAO Penman-Monteith ET_o equation to account for the effects of elevated atmospheric CO₂ concentration and increase in temperature on ET_o.

In this study, the connections between anticipated climate change and hydrologic processes were studied to better understand the climate change impact on irrigation water requirements for two major crops in the islands of Hawai'i. The specific objectives of this research are to evaluate the potential impact of climate change on: (i) irrigation water requirements for a short cropping season annual crop, seed corn, and a perennial crop, coffee; and (ii) the major water cycle components, e.g., evapotranspiration, canopy interception, runoff, and excess water losses/groundwater recharge below the rootzone under Hawaiian conditions.

Climate change sensitivity assessment was performed based on long-term historical weather data collected at Honolulu International Airport (Station ID–GHCND: USW00022521) on the Island of Oahu, Hawai'i, USA. The latitude and longitude of this station are 21.323 ° and −157.929 °, respectively. The station is located at an elevation of 2.1 m AMSL. Daily climate data (precipitation, wind speed, maximum and minimum temperature) from 1984/01/01 to 2010/10/31 (27 years) were downloaded from the Global Historical Climatology Network (GHCN) of the National Climatic Data Center on the National Oceanic and Atmospheric Administration's (NOAA) website (http://www.ncdc.noaa.gov/cdo-web/).

The IManSys is a numerical simulation model; it calculates irrigation water requirements (IRRs) for different crops based on the water balance approach and using site-specific data and historical weather data. Water budget models have been used for irrigation scheduling and crop water requirement estimation (Smajstrla & Zazueta 1988; Obreza & Pitts 2002; Fares et al. 2008). Smajstrla & Zazueta (1988) reported on the Agricultural Field Scale Irrigation Requirements Simulation (AFSIRS) model that uses a water balance approach with a layered soil column to simulate soil water infiltration, redistribution, and extraction by evapotranspiration as steady state processes on a daily basis. The AFSIRS model simulates the irrigation requirements for a crop based on plant physiology, soil, irrigation system, growing season, climate and basic irrigation management practice. The AFSIRS does not account for runoff neither for canopy interception nor does it multiple crops. It addition, it cannot calculate ET based on other weather data, nor does it have databases for soils other than those found in Florida. Another major deficiency is that AFSIRS does not account for climate change effects on IRRs calculation. Based on the deficiencies of the current water balance models, e.g. AFSIRS, it was necessary to develop a new model that builds on the water balance approach used by AFSIRS and offers new functions needed to calculate IRRs across different climates and environmental conditions where runoff, canopy interception, and climate change are important. Such a new model should also be able to calculate ET data.

Thus, IManSys was developed based on AFSIRS’ approach (Smajstrla & Zazueta 1988); in addition to IRRs calculations, IManSys calculates runoff, canopy interception, effective rainfall, ET and IRRs for multiple crops based on crop growth parameters, soil properties, irrigation system efficiency, and long-term weather data (precipitation and evapotranspiration). IManSys accounts for climate change effects and water management practices on IRRs. If ET data is not available, IManSys calculates evapotranspiration using limited weather data, e.g. temperature-based model ETM (Fares & Mansell 1996) or using complete weather data, e.g. FAO Penman-Monteith method (Allen et al. 1998). IManSys uses multiple databases, e.g., soil and plant growth parameters, irrigation system efficiencies, and canopy interception. IManSys was implemented in JAVA object oriented language. IManSys output includes detailed net and gross IRRs, and all water budget components at different time scales (daily, weekly, biweekly, monthly, and annually) based on non-exceedance drought probability which is calculated from a conditional probability model that uses the type I extreme value distribution for positive non-zero irrigation values (Fares & Fares 2012).

IManSys’ simulated soil profile depth is assumed to be equal to the crop root zone depth. The plant root zone is divided into irrigated (upper 50%) and non-irrigated (lower 50%) zones based on the common practice of irrigating only the upper portions of the crop root zone where most of the roots are located (Smajstrla 1990; Smajstrla & Zazueta 1988). It is assumed that 70% and 30% of crop ET are extracted from the irrigated and non-irrigated zones, respectively, when water is available (SCS 1970). This pattern of water extraction is typically assumed for well-irrigated plants on non-restrictive soil profiles (Smajstrla & Zazueta 1988).

The water storage capacity is the amount of water that is available for plant uptake. It is calculated as the equivalent water between field capacity and permanent wilting point for a given soil multiplied by the depth of the root zone. Irrigation is scheduled based on an allowable level of soil water depletion from the root zone. Irrigation amounts are calculated to restore the soil water content to field capacity.

The irrigation requirement is calculated as the depth of water required to replenish the soil water content to field capacity in the irrigated crop root zone. Irrigation events were assumed to start when the available water for plant uptake decreases to a predetermined minimum allowable level, termed allowable soil water depletion (AWD) percentage. Allowable soil water depletion values were determined from the literature and are fractions of the available soil water storage capacity that can be allowed to be depleted without significant reduction in crop yield. Allowable soil water depletion values for the annual and perennial crops are user specific and can be provided with crop information. The model uses AWD equals to 0.50 as the default value for all perennial crops. A value of 0.50 means that 50% of the available water in the irrigated crop root zone is allowed to be depleted between two consecutive irrigation events.

More details about the IManSys model can be found in Fares (2008), Fares (2009), and Fares & Fares (2012). The theoretical and computational methods used in IManSys are the same as those used in IWREDSS (Fares 2008). Details about the IWREDSS model and its validation are given in Fares (2008).

Different methods to calculate potential evapotranspiration were implemented in IManSys. The FAO Penman-Monteith equation was modified to incorporate climate change scenarios of different CO₂ emissions and temperature rises in future ET calculations.

The ratio r_s/r_a in the FAO Penman-Monteith equation is a function of wind speed at 2 m height (0.34 u₂). We modified the value of ratio r_s/r_a according to variations in CO₂ concentration. An increase in CO₂ concentration not only causes increases in air temperature but also will result in a reduction of the leaf stomatal conductance (Saxe et al. 1998; Medlyn et al. 2001; Wullschleger et al. 2002) and an increase in LAI because of enhanced photosynthesis (Pritchard et al. 1999; Wand et al. 1999). The stomatal conductance of reference grass surface is adjusted using a similar approach to that used in the Soil and Water Assessment Tool (SWAT) (Arnold et al. 1998). Similar to Eckhardt & Ulbrich (2003), we used a smaller increase (7%) in LAI with a doubling of CO₂ concentration.

The remaining terms are estimated; following Allen et al. 1998 using observed or increased daily maximum and minimum temperature in different scenarios.

In this study, a total of 24 climate change scenarios (Table 2) were generated on the basis of the IPCC Special Reports on Emission Scenarios (IPCC 2001), local trend analysis and GCM downscaling (Timm & Diaz 2008) similar to the climate change scenarios used by Ficklin et al. (2009) and Safeeq & Fares (2012). On the basis of the IPCCs projections of greenhouse gas emissions for the 21st century, atmospheric concentration of CO₂ is expected to increase between 550 (B1 emission scenario) and 970 ppm (A1F1 emission scenario) from its reference emission scenario of 330 ppm. The two extreme scenarios represent a future world of a very rapid economic growth (A1F1 emission scenario–970 ppm) or a future world with low economic growth and fossil fuel independency (B1 emission scenario–550 ppm).

To bracket the range of changes in precipitation over the Hawaiian Islands, four arbitrary precipitation scenarios with respect to the reference level (i.e., 0, ±5, ±10 and ±20%) were selected. Two additional precipitation-based scenarios were generated on the basis of six GCM predictions for Hawai'i under AR4 A1B emission scenario. Temperature-based scenarios were generated by increasing the temperature by 1.1 and 6.4 °C. All the climate change scenarios were generated using the ‘delta method’ (Hamlet & Lettenmaier 1999) on observed precipitation and air temperature data from the Honolulu Airport Weather Station for the period between 1984 and 2010. Daily precipitations were increased or decreased relative to the original observed data; thus, the latter data were multiplied by a scaling factor. A scaling factor with a value of 1.05 means a 5% increase in precipitation compared with historical data. One of the reasons for choosing two cropping seasons for the annual seed corn crop is to cover the potential impact climate change might have on rainfall distribution across the year. Changes in air temperature were made by adding the required 1.1 or 6.4 °C to observed temperature, half of each of these values is added to the maximum daily temperature and the other half to the minimum daily temperature.

Different scenarios were simulated using IManSys for 27 years after adjusting precipitation, temperature, leaf conductance, and LAI. Other climate variables (i.e., solar radiation, wind speed, and relative humidity) were kept constant for all scenarios. Seed corn growing seasons and their corresponding lengths are given in Table 3. The growing seasons used for all simulated scenarios are the same for both crops. The dry and wet seasons for corn start in June and December, respectively. The irrigation system used in all simulations is the drip irrigation system which has an irrigation efficiency of 0.85. Simulation results are presented on a monthly and an annual basis; they were then compared with those of the reference scenarios.

Based on an extensive review of the literature, the authors did not find any previous studies on the potential impact of climate change on IRRs of crops in Hawai'i; however, there were other studies under non-tropical environments. Projections of future irrigation requirement for field corn in Georgia, USA calculated using a climate change scenario derived from the HadCM2 (Johns et al. 1997) global circulation model showed a decrease in IRRs by 20% by 2030 and 50% by 2090 due to the combined effect of precipitation increase and shortening of the growing seasons (Hatch et al. 1999). Hatch et al. (1999) used the Priestley-Taylor equation to calculate reference evapotranspiration; they also incorporated the effect of elevated CO₂ on stomatal closure on potential transpiration using the ratio (Peart et al. 1989) of transpiration under elevated CO₂ conditions to that under ambient conditions. In general, results of the current study concur with their prediction of an IRR decrease. However, results of IRRs across the US corn-belt, under three climate change scenarios showed mixed result (Strzepek et al. 1999). In their study they also used a similar approach to calculate ET_o; the calculated potential transpiration reflects the effects of elevated CO₂. Allen et al. (1991) studied the effects of CO₂-induced climatic changes on IRR in the Great Plains region (Nebraska, Kansas, Oklahoma and Texas) using GCMs data and a water balance model. They also used the Penman-Monteith equation to calculate ET_o; they simulated the effect of elevated CO₂ level transpiration by increasing the value of reference crop bulk stomatal resistance (+20, +40, +60 and +80%). They reported a decrease in IRR for corn at all locations for increased bulk stomatal resistance of 40–80% under the GISS (Goddard Institute for Space Studies) scenario; however, IRR increased at all locations for increased bulk stomatal resistance of 20–60% except for Texas under the GFDL (Geophysical Fluid Dynamics Laboratory of Princeton University) scenario. The IRRs may increase or decrease depending on the spatial location of the study area and climate change scenarios used.

This work investigated the impact of potential future climate change scenarios on the gross IRRs of two major agricultural crops (corn and coffee) in Hawai'i and the subsequent water budget components (runoff, drainage, and canopy interception) using IManSys.

It was found that an increase in temperature resulted in an increase in ET demand and consequently in an increase of IRR for both crops; however, IRR is higher for coffee (>10%) than for that of seed corn. As expected, gross IRR is negatively correlated with precipitation; however, there is a direct correlation between drainage and precipitation. Irrigation water requirement decreased with increased CO₂ emission. Results of the study indicated that the impact of climate change on two major crops grown in a major Hawaii Island (seed corn and coffee) is very sensitive to future climate scenarios generated as predicted by different GCMs. The average percentage decrease in IRRs for coffee is 6.4% and 42.2% for CO₂ emission levels of 550 ppm and 970 ppm, respectively. Whereas the corresponding average decrease in IRRs for seed corn are 8% and 50.1%, respectively. The average percentage decrease in IRR for seed corn is comparatively higher than that of coffee. Runoff increased with a precipitation rise, but it did not show any change in response to CO₂ level variations. Canopy interception and drainage increased with increased CO₂ emission. However, the magnitude and the rate of drainage increase are higher for seed corn than for coffee. Evapotranspiration increased for rising temperatures, and as a result the gross irrigation water requirement was also increased. Changes in rainfall variability and increased evaporation will directly impact rain-fed agriculture and reduce water availability for irrigation.

Results of this study as well as those obtained from other studies suggest that as the CO₂ content in the atmosphere continues to rise, terrestrial plants will likely display reductions in transpirational water loss. As a result, most plants might be able to better deal with periodic water shortages and warmer temperatures, possibly even could expand their ranges into areas where it was too dry for them to successfully live and reproduce in the recent past.

This study only focused on IRR calculations for two main crops grown on Hawai'i. Further studies on predicting crop yield responses to these different irrigation scenarios using crop modeling are essential to quantify the impact of climate change on the yield of these two crops in Hawai'i.

This project was partially supported by USDA National Institute of Food and Agriculture, Hatch Act Formula Grant in University of Hawai'i. Partial funding was also provided by the National Institute of Food and Agriculture of the United States Department of Agriculture Evans-Allen funds in Prairie View A&M University. The authors wish to thank Kyle Mulleady for his assistance in updating the IManSys program. The authors also wish to thank Hla Htun for his assistance in the early stages of this study.