Determination of Crop Water Requirements for Maize in Abshege Woreda, Gurage Zone, Ethiopia

In Ethiopia where crop production overly depends on rainfall and temperature, studying the variability of these climate variables at a local scale is essential to devise proper strategies that enhance adaptive capacity. In light of this, a study was conducted in Abshege Woreda, Gurage Zone to determine crop water requirement of maize, which is major food crop of the area. Ten years i.e., (2006-2015) Indibir station climatological records of (sunshine duration hr/day), maximum and minimum temperature (OC), humidity (%) and wind speed (km/day) at 2 meters height were used in FAO Penman Monteith method. Secondary data were used to collect important soil parameters required for determination of crop water requirement in the study area such as field capacity (FC), permanent wilting point (PWP), initial soil moisture depletion (as % TAM) and available water holding capacity (mm/meter) while data for maximum rain infiltration rates (mm/day) and maximum rooting depth (cm) were obtained from literature based on similar textural class of the soil in the study area. Lengths of total growing periods of the crop was determined from ten years climate data and planting date was 10th May acquired from OAWBA and farmers of the area. Crop coefficients (kc), rooting depth, depletion level and other agronomic parameters were obtained from FAO guidelines (No 56) for each growth stage. The analyzed data indicated that Crop water requirement was estimated using CROPWAT 8.0 for window. A maize variety with a growing period of 140 days to maturity would requires 423 mm depth of water, while 101 mm would be required as supplementary irrigation depth. Citation: Abirdew S, Mamo G, Mengesha M (2018) Determination of Crop Water Requirements for Maize in Abshege Woreda, Gurage Zone, Ethiopia. J Earth Sci Clim Change 9: 439. doi: 10.4172/2157-7617.1000439


Introduction
The amount of water required to compensate for the evapotranspiration loss from the cropped field is defined as crop water requirement. The values for crop evapotranspiration and crop water requirement are therefore, identical (except opposite sign), crop water requirement refers to the amount of water that needs to be supplied (positive sign), while crop evapotranspiration refers to the amount of water that is lost through evapotranspiration (negative sign). The experimentally determined ratios of ET c /ET o , called crop coefficient, is used to relate crop evapotranspiration under standard conditions (ET c ) to ET o i.e., reference evapotranspiration, which is an atmospheric parameter. The net irrigation water requirement represents the difference between the crop water requirement and effective rainfall.
The CROPWAT model is widely used to estimate ET o and ET c . In actual fact, CROPWAT is a computer program for irrigation planning and management, developed by FAO [1]. However the same principle holds for rain-fed crop production as well. Its basic function includes the calculation of reference crop evapotranspiration, crop water requirement and scheme irrigation requirement. Although several methods exist to determine ET o , the Penman Monteith method has been recommended as the appropriate combination method to determine ET o from climatic data on: temperature, humidity, sunshine and wind speed are required [2].
The highest value of ET o is experienced in areas that are hot, dry, windy and sunny. In many cases it is possible to obtain estimates of ET o for a given locality from the meteorological service [3]. Study conducted at Arba Minch by Mekonen, a maize variety with a growing period of 135 days to maturity would require 535.8 mm depth of water, while 307.2 mm would be required as supplementary irrigation depth with effective rainfall of 259.6 mm during the period of January to May 1 st . In rain-fed cropping, the estimated crop water requirement is an important parameter to use alternate practices like mulching, reducing plant population by certain percentage in order to reduce the stiff competition and many more others.

Objective
The objective is to determine crop water requirements for maize production.

Methodology Location
The study was conducted in Abshege Woreda, Gurage Zone of SNNPRS, Ethiopia ( Figure 1). The geographical coverage of the study area is 8° 17' 22.6" N and 37° 46' 55.98" E at an elevation range of 1050 to 1883 m.a.s.l. It has a total area of about 57313.85 ha of which 40119.7 ha is cultivated.

Estimation of crop water requirements for maize production
Secondary data were used to collect important soil parameters required for determination of crop water requirement in the study area such as field capacity (FC), permanent wilting point (PWP) and initial soil moisture depletion (as % TAM). Available water holding capacity/ total available soil moisture (mm/meter) was determined from FC and PWP, while data for maximum rain infiltration rates (mm/day) and maximum rooting depth (cm) were obtained from literature based on similar textural class of the soil in the study area [4].

Abstract
In Ethiopia where crop production overly depends on rainfall and temperature, studying the variability of these climate variables at a local scale is essential to devise proper strategies that enhance adaptive capacity. In light of this, a study was conducted in Abshege Woreda, Gurage Zone to determine crop water requirement of maize, which is major food crop of the area. Ten years i.e., (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) Indibir station climatological records of (sunshine duration hr/day), maximum and minimum temperature (OC), humidity (%) and wind speed (km/day) at 2 meters height were used in FAO Penman Monteith method. Secondary data were used to collect important soil parameters required for determination of crop water requirement in the study area such as field capacity (FC), permanent wilting point (PWP), initial soil moisture depletion (as % TAM) and available water holding capacity (mm/meter) while data for maximum rain infiltration rates (mm/day) and maximum rooting depth (cm) were obtained from literature based on similar textural class of the soil in the study area. Lengths of total growing periods of the crop was determined from ten years climate data and planting date was 10 th May acquired from OAWBA and farmers of the area. Crop coefficients (k c ), rooting depth, depletion level and other agronomic parameters were obtained from FAO guidelines (No 56) for each growth stage. The analyzed data indicated that Crop water requirement was estimated using CROPWAT 8.0 for window. A maize variety with a growing period of 140 days to maturity would requires 423 mm depth of water, while 101 mm would be required as supplementary irrigation depth.   2015) Indibir station climatological records of (sunshine duration hr/ day), maximum and minimum temperature (OC), humidity (%) and wind speed (km/day) at 2 meters height were used in FAO Penman Monteith method [5].

Results and Discussion
Reference crop evapotranspiration for maize crop grown in the area was estimated from long term climate parameters (2006-2015) in CROPWAT 8.0 software. In the study area there is only 10 years long term climatological data on sunshine hours, humidity and wind speed. Therefore, in estimating ET o , 10 years Indibir station climatological records of sunshine hours, maximum and minimum temperature, humidity and wind speed at 2 meters height were used in CROPWAT 8.0 software. Mathematical Penman Monteith form of the combination equation is as follows: Where, ET o is reference crop evapotranspiration (mm/day), R n is net radiation at the crop surface (MJ m -2 day -1 ), G is soil heat flux density (MJ m -2 day -1 ), T is mean daily air temperature (OC) at 2 m height, es is saturation vapour pressure (kPa), ea is actual vapor pressure (kPa), es-ea is saturation vapor pressure deficit (kPa), ∆ is slop vapor pressure curve (kpa/OC), U 2 is the average wind speed at 2 m height (m/s) and γ is psychrometric constant (kPa/OC).
In addition to meteorological data used to compute the ET o , crop related data is required to estimate the crop water requirements on daily, decadal and monthly bases. Crop water requirements were computed for maize crop which is widely grown as a food crop in the study area. Lengths of total growing periods of the crop was obtained from OAWBA and farmers of study area and again the planting date was 10 th May acquired from OAWBA and farmers of the area. Crop coefficients (k c ) defined as the ratio of the crop evapotranspiration (ET c ) of a disease free crop grown in a large field adequately supplied with water to the reference crop evapotranspiration (ET o ), rooting depth, depletion level and other agronomic parameters were obtained from FAO guidelines (No 56) for each growth stage [6]. A crop coefficient (k c ) for maize is 0.3 for initial, 0.8 developments, 1.2 flowering and irrigation in the study area, and 101 mm depth of water is required as supplementary irrigation for maize crop grown in the area during dry spells and drought for an individual farmer to offset the effect of water stress on maize yield. Therefore, absence of supplementary irrigation during dry spells would result in reduced maize yield. A similar study undertaken at Arba Minch by Mekonen also reported that a maize variety with a growing period of 135 days to maturity requires 535.8 mm depth of water and to meet this 307.2 mm is required to be given as supplementary irrigation.
The current irrigation volume of the study Woreda is 957 m 3 , which is irrigate only 0.95 ha of land which is less than one hectare. Hence, inline of the present in climate change water is becoming scarce, and the administration of the area should work to expand irrigation, and water harvesting technology in order to reduce an incoming impacts of climate change and variability. Provision of irrigation is likely to play a strategic role in either stabilizing the production of grains or in supporting a low risk, high value production system with a strong commercial focus during rainfall shortage and prolonged dry spell.

Conclusion
Crop water requirement of maize also showed that a maize variety with a growing period of 140 days to maturity requires 423 mm depth of water, to need this 101 mm required as supplementary irrigation to overcome the observed deficit in rainfall.
in the month of July 2012 (Table 2). Lowest ET o s were recorded during kiremt season, in the month of July (74.5 mm) followed by August (77.8 mm) and June (85.2 mm), while maximum monthly ET o occurred in the month of March (147.4 mm) followed by April (136.5 mm) and January (133.4 mm). Monthly SD and CV of ET o varied from 7.6 mm to 17.6 mm and 6.9% to 19.5%, respectively (Tables 1 and 2).