Irrigation contribution/importance in crop production
Corn production in Nebraska is irrigated.
Irrigation increases yield because of the water supply and cooling effect.
irrigation cools canopy temperature and reduce heat stress \cite{Siebert_2014}. through evaporative cooling
Irrigation cooling effect \cite{Bonfils_2007} irrigation causes large reductions in surface daytime temperatures.
irrigation mitigates heat stress and hot weather.
climate condition affects farms decision of irrigation \cite{Knapp_2017}
Objective:
How vegetation properties, yield differs between rainfed and irrigated crops.
Data and methods
Data
- The MODIS 8-day LST (MYD11A2) and 16-day EVI (MYD13Q1) of Collection 6 from 2003 to 2016 at spatial resolutions of 1 km and 250 m, respectively. The daytime and nighttime LST from "MYD11A2" product approximate the maximum and minimum temperature of a day, as the Aqua satellite has a local overpass time of 13:30/1:30.
- PRISM climate data, averaged to county-level, including rainfall and temperature.
- The 2005 Nebraska irrigation map provides an field-level inventory of active center pivot and other irrigation systems in Nebraska for the 2005 growing season. The irrigation systems are identified using Landsat 5 satellite and Farm Service Agency Ortho Imagery (see more information at https://calmit.unl.edu/metadata-2005-nebraska-land-use-center-pivots-irrigation-systems).
- The 30 m maize mask extracted from Crop Data Layer (CDL) from 2003 to 2016 in Nebraska.
- The county-level irrigated and rainfed yield data for maize from 2003 to 2016 in Nebraska, obtained from NASS (https://quickstats.nass.usda.gov/). How many counties?
- Daytime air temperature measurements of three Nebraska flux sites of maize (US-Ne1, Ne2, and Ne3) from 2001 to 2013 (gap-filled), the "TA_F_MDS" variable. NE1 is an irrigated maize site. NE2 is also an irrigated site but with maize and soybean rotation (maize in odd years during 2001–2009 and all years from 2010 to 2013). NE3 is a rainfed site with maize and soybean rotation (maize in all odd years during 2001–2013). Since these three sites are close in distance, assuming they have similar environmental conditions, the differences between irrigated and rainfed pairs such as Ne1−Ne3 and Ne2−Ne3 are indicative of the irrigation effect.
Method
MODIS data processing (on google earth engine).
- In this study, the irrigation effect is quantified as the differences in the properties (e.g., yield, LST, and EVI) between irrigated and rainfed crops at the county-level. While the yield of irrigated and rainfed in each county are readily available from NASS, the LST and EVI for irrigated and rainfed maize in each county have to be extracted with the procedures described below on google earth engine (Fig. \ref{226425}):
- The irrigation map was first overlaid with the 2005 CDL data to extract irrigated and rainfed maize pixels at 30 m resolution. These 30 m pixels were then aggregated to the MODIS resolutions with the majority method (1km for LST and 250 m for EVI 250m) to create the irrigated and rainfed maize mask. The resulting masks were applied to MODIS LST/EVI data to extract LST/EVI of irrigated and rainfed maize and their differences were computed in counties where both present.
- Through above produces, yield, LST, and EVI of irrigated and rainfed maize in each county were obtained, but only for 2005, because the irrigation map is developed only for that specific year. In order to obtain data for more years, we assumed the irrigation map, which is produced for 2005, is also applicable to other years. The validity of this assumption was tested in SI by comparing the irrigated and rainfed maize area derived with this assumption and those county-level observations from NASS. The high correlations of these two from 2003 to 2016 for irrigated (0.99) and rainfed maize area (0.94) shown in Figs \ref{848838} and \ref{947083} support the credibility of this assumption.