Estimation of Crop Water Requirements in Irrigated Land of the Khorezm Oasis in the Context of Climate Change

Khorezm province is located in the northwest part of Uzbekistan in the basin of one of largest water sources – the Amu Darya River and occupies the left bank in the Amu Darya lower reaches. The area of the province is 6,100 km2. The province borders Karakalpakstan in the North, Turkmenistan in the South, and Bukhara province of Uzbekistan in the South-East. Uzbekistan is situated in the territory, where high rates of climate change are expected and observed. According to forecasts, further climate change would cause even higher air temperatures, altered precipitation patterns and severe and prolonged droughts, with consequent lowering of available water resources. More plausible scenarios for Uzbekistan suggest more than 4°С rise in average annual air temperatures by 2080. Water discharge along the Amu Darya River is expected to decrease potentially by 10-15%. Objective of given work is to analyze and assess the positive impacts of climate change through alterations of bioclimatic potential in given terrain and agromelioration parameters of crops, with consequent changes in crop water requirements. Earlier research results showed that the observed growth of thermal potential allows earlier sowing and more rapid accumulation of effective temperatures. This will shorten plant development phases, on the one hand, and, as a result, reduce water use by crops, on the other hand.


Introduction
Uzbekistan is exposed heavily to climate change threats.The UzHydromet (Chub, 2007) forecasts that climate change would further lead to rising temperatures, changes in precipitation patterns, and more severe and prolonged droughts, with consequent worsening of water availability.The most probable scenarios for Uzbekistan imply the rise in average annual temperature by more than 4°C by 2080.It is predicted (Dukhovniy, 2008) that the flow may decrease by 2-3% in the Syrdarya Basin and by 10-15% in the Amudarya Basin.Hence, there is a need for serious efforts to be made for adaptation to climate change and mitigation of risks.
Present research was carried out as part of the LaVaCCA Project entitled, "Assessing Land Value Changes and Developing a Discussion-Support-Tool for Improved Land Use Planning in the Irrigated Lowlands of Central Asia" supported by Volkswagen Foundation.This work is a follow-up of multi-year author's research on climate change and adaptation (Stulina & Madramootoo, 2005;Stulina & Usmanov, 2002;Stulina & Solodkiy, 2011).Karamazy-Klychbai, Tashsaka, and Palvan-Gazawat.
The inter-farm network management is assigned to WUA that serve (as of 2014) 4755 water users.
The climate is sharply continental, with the summer temperature reaching + 45 0 С and the winter temperature falling down to -32 0 С.Given thin snow cover, the soil freezes down to half a meter.Shallow saline groundwater (0-1 m) causes salinization over the vast area.Cropping patterns are shaped by natural-climatic conditions.The territory of the province is adjacent to the northern boundary of land where cotton can be grown.Shallow groundwater forms hydromorphic soil, which is favorable for rice cultivation.Cucurbits, such as melon and water melon are also common there.

Research Methods
Water requirements were modeled using the REQWAT model (Stulina & Usmanov, 2002) developed on the basis of the CROPWAT model, which underwent certain modifications.The main difference from the CROPWAT is the module of groundwater contribution.
Climate data, a soil map, a groundwater well location map and the observations data on groundwater levels for a series of years are used as input for the calculations.The results are displayed in form of ten-day or monthly crop water requirements averaged for the studied area or as maps of annual crop water requirements for various crops.
The REMO model (Mannig et al., 2013;Parry et al., 2007;Parry et al., 1999;Rozenzweig et al., 2001;Vleck et al., 2004) was used for the forecasts of climate change.This climatic model is based on the ECHAM 5 model developed at the Max Plank Institute (Germany).That is the model of global atmosphere circulation.It is used for calculation of global and regional patterns of climate change.The A1B scenario of average warming as a result of greenhouse gas emission was played in the model.Given model allowed constructing the artificial temperature and rainfall series until 2080.
With the adoption of the food security doctrine, the wheat areas extended significantly by integrating wheat into the crop rotation.(Figure 2, Figure 3).
Unfortunately, the expansion of wheat area by 15% from 1980 till 2012 was accompanied by 19% reduction of the forage area (Figure 2, Figure 3).Those years, the reductions in yields were 30% for cotton, 13% for wheat, 65% for rice, and 20% for forage crops against the average yields over the last 30 years.

Crop Water Requirements
Water delivery per irrigated hectare also depends on climatic conditions.
Water availability dramatically drops during dry years.Figure 6 shows the catastrophic water situation in the years of drought, when water delivery decreased from 4,000 million m 3 to 900 million m 3 .The decrease in unit water delivery in m 3 /t is a result of reduction of the total available water (Figure 6) and the cause of lowering yields (Figure 5).

Assessment of Thermal Resources and Their Forecast
The Khorezm oasis situated in the north of the Republic refers to the cool thermal zone, with the sum of effective temperatures as 1000 -2800 о С, according to Babushkin's classification (Babushkin, 1974).The thermal capacity is estimated through the sum of effective temperatures.The effective temperature is the difference between the mean daily temperature and the temperature at which development of any crop starts.This threshold temperature is specific for each crop (Table 1).The increase in thermal resources reduces the risks for growing of heat-loving crops, such as cotton.

Changes in the Date of Steady Temperature Transition
It is clearly observed that the time suitable for sowing comes earlier.In the REMO scenarios, as compared to the base scenario, steady transition of air temperature through 10 о С is 4 days earlier and closer to winter and 2 days in the last days of the growing season by 2030, 3 days earlier by 2050, and 6 days earlier in spring season by 2080 (Figure 9).

Length of Crop Growing Season
Thermal resources are to provide annual ripening of different crop varieties cultivated in the region.
The climate change and temperature rise will lead to shift in the time of accumulation of the total effective temperature, which is necessary for ripening of crops.
Table 2 shows the sums of effective temperatures needed for plant development phases and for complete ripening till harvesting.The comparison of historical data with REMO simulations indicates to future changes in processes of plant growth and development.Subsequently, this will entail modification of the whole agronomic cycle.
There are 4 phases of plant development, which are described below (Table 3).
Table 3. Description of plant development phases I-phase P-1 starting with sowing, sprouting till formation of cotyledonous leaves and, probably, two or three full-fledged leaves; soil surface is not covered or slightly covered with plants (canopy is less than 10%) II-phase P-2 crop development (from the beginning of quick growth till blossoming, when the rate of development slows down; by the end of the phase canopy is 70-80%), III-phase P-3 flowering or mid-season phase (includes the main period of blossoming and early phase of seed and fruit formation; from effective canopy 70-80% till start of fading and falling off IV-phase P-4 ripening (when crop develops, ripens and harvest is collected) According to this classification and using the sum of effective temperatures needed for each phase to be completed, the length of crop development phases was calculated for main crops and for different years and relevant forecast for the future was made.
The analysis made for cotton demonstrates the results.Figure 10 shows changes in development phases of mid-season cotton, whereas Table 4 shows changes in the length of development phases for early ripening, midand late-season cotton varieties.The growing season of most crops becomes shorter as well.
Based on the analysis made above, the temperature rise has a positive effect through acceleration of physiological processes and, thus, shortening of the length of crop development and growth.However, with temperature rise, the risk of extremes affecting adversely agricultural production becomes high.The rise of temperature above critical value will provoke plant stress.When analyzing water requirements of crops, we took into account probable changes in agro-climatic parameters, including corrected sowing dates, dates of crop development phases, related crop coefficients, and total length of growing seasons.The Table above shows the results of assessment of potential changes in crop water requirements in the Khorezm oasis, based on REMO's climate scenarios.

Conclusion
As a northernmost area, where cotton, the main crop in the region, is produced, the Aral Sea basin lacks sufficiently sustainable conditions for growth and secured yields of cotton everywhere.However, the temperature rise means that the northern territories start showing climate characteristics that are more typical for southward territory, and the area where potentially cotton could be grown extends.More prolonged growing season offers the possibility to have two harvests a year.Accordingly sowing dates will change.Increased temperatures and carbon dioxide concentrations have a positive effect on growth and development of plants.
Given that all main inputs, nutrients, water, and pest and disease control are in place, yield potential of some crops becomes higher under climate change.Thus, the changing climate forces agriculturalists to revise farming practices as with the rise in temperature and moisture and the change in river flow, the whole process chain needs to be modified.

Figure 4 .
Figure 4. Cropping patterns in the Khorezm province

Figure 5 .
Figure 5.Yields of main crops in the Khorezm province

Figure 7 .
Figure 7. Annual sum of effective temperatures higher than 10 degrees, Khorezm planning zone

Figure 9 .
Figure 9. Changes in the dates of steady transition through 10 о С, Khorezm

Figure 10 .
Figure 10.Change in the length of development phases for mid-season cotton, by year

Figure 11 .
Figure 11.Trend of the length of mid-season cotton growth

Figures 14 ,
Figures 14, 15 and 16 show the results of water requirement calculations by the REQWAT model using the REMO's climate forecasts, for different cotton varieties.Water requirement trends indicate to lowering of cotton irrigation requirements.However, this takes place only at the expense of shortened growing season of cotton as shown above (Table.4, Table.5; Figure 11, Figure 12, Figure 13).

Figure 14 .
Figure 14.Scenario of changing water requirements of early ripening cotton under climate change

Table 2 .
Sums of effective temperatures above 10 о by crop development phase under appropriate soil moisture conditions