Groundwater (GW) management is an essential element in irrigated agriculture. This paper analyzes the temporal dynamics of GW table and salinity in Khorezm, a region of Uzbekistan which is situated on the lower Amu Darya River in the Aral Sea Basin and suffering from severe soil salinization. We furthermore identify the critical areas for potential soil salinization by examining GW table and salinity measured during 1990-2000 in 1,972 wells, covering the entire region. Additionally, case studies were performed to assess the contribution of the GW to the soil salinization on a field scale. Over the entire area, GW was only moderately saline averaging 1.75±0.99 g l −1 However, GW levels were generally very shallow averaging 148±57 cm below the ground surface and thus likely to prompt secondary soil salinization. Three case studies where GW table, soil and GW salinity were closely monitored at the field scale, suggested that the elevated GW levels forced soil salinization by annually adding 3.5-14 t ha −1 of salts depending on the position and salinity of the GW table. Maps interpolated from the regional dataset revealed that GW was significantly shallower and more saline in the western and southern parts of Khorezm despite the presence of a drainage network which is rather uniformly distributed throughout the region. The results of the current study will assist the development of an improved drainage management in Khorezm.

Advancing land degradation in the irrigated areas of Central Asia hinders sustainable development of this predominantly agricultural region. To support decisions on mitigating cropland degradation, this study combines linear trend analysis and spatial logistic regression modeling to expose a land degradation trend in the Khorezm region, Uzbekistan, and to analyze the causes. Time series of the 250-m MODIS NDVI, summed over the growing seasons of 2000–2010, were used to derive areas with an apparent negative vegetation trend; this was interpreted as an indicator of land degradation. About one third (161,000 ha) of the region’s area experienced negative trends of different magnitude. The vegetation decline was particularly evident on the low-fertility lands bordering on the natural sandy desert, suggesting that these areas should be prioritized in mitigation planning. The results of logistic modeling indicate that the spatial pattern of the observed trend is mainly associated with the level of the groundwater table (odds = 330 %), land-use intensity (odds = 103 %), low soil quality (odds = 49 %), slope (odds = 29 %), and salinity of the groundwater (odds = 26 %). Areas, threatened by land degradation, were mapped by fitting the estimated model parameters to available data. The elaborated approach, combining remote-sensing and GIS, can form the basis for developing a common tool for monitoring land degradation trends in irrigated croplands of Central Asia.

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Irrigated croplands in Central Asia are highly prone to land degradation due to their environmentally fragile physical settings and intensive agricultural practices. This study: (i) assesses the state of croplands in irrigated areas in northern Uzbekistan, based on the time series of MODIS-NDVI imagery; (ii) analyzes relationships between the identified trend of cropland degradation and soil quality, terrain characteristics, population density, and land use; and (iii) synthesizes the results which form the basis for recommendations on spatial targeting of land rehabilitation measures. The NDVI-based cropland degradation assessment revealed a significant decline of cropland productivity across 23% (94,835 ha) of the arable area in the study region between 2000 and 2010. We conclude that the degraded cropland identified within areas of high population density and with better quality soils, can be prioritized for rehabilitation measures. For degraded croplands located in sparsely populated areas with poorer quality soils, other alternatives (such as leaving cropland fallow) may be more effective depending on the severity of degradation and economic viability of rehabilitation options. Zusammenfassung: �ew�sserte Anbau� �chen in �entralasien zeigen eine starke Anf�lligkeit f�r �odendegradation. Ur-�ew�sserte Anbau��chen in �entralasien zeigen eine starke Anf�lligkeit f�r �odendegradation. Ursachen hierf�r sind physische Umweltein��sse sowie intensive Landwirtschaft. Diese Studie: (i) beurteilt den �ustand der bew�sserten, landwirtschaftlich genutzten Fl�che im nördlichen Usbekistan, aufgrund einer �eitreihe von MODIS-NDVI �ildern; (ii) sie setzt die ermittelte �odendegradationsentwicklung in �eziehung zu �odenqualit�t, Gel�ndeeigenschaften, �evölkerungsdichte, Landnutzung und (iii) stellt die Ergebnisse so dar, dass Empfehlungen zu �odenrehabilitierungsmaßnahmen f�r verschiedene Gebiete abgeleitet werden können. Eine NDVI-basierte �ewertung der �odendegradation zeigte einen signifikanten R�ckgang der Leistungsf�higkeit von Acker��chen von 23% (94.835 ha) auf den Anbau��chen des Untersuchungsgebiets zwischen 2000-2010. Degradierte Acker��chen, in Gebieten mit hoher �evölkerungsdichte und �öden besserer Qualit�t, sollen f�r Rehabilitationsmaßnahmen vorgeschlagen werden. F�r die degradierten Fl�chen, die in sp�rlich besiedelten Gebieten mit weniger guter �odenqualit�t liegen, muss entschieden werden, ob die landwirtschaftliche Nutzung eingestellt wird. Dieses h�ngt von der Schwere der Degradation sowie der Wirtschaftlichkeit der Rehabilitationsmaßnahmen ab.

Extensive degradation of irrigated croplands, due to increasing soil salinity and depletion of soil nutrient stocks, is a major problem in Central Asia (CA), one of the largest irrigated areas in the world. To assess the potential for improving the productive capacity of degraded lands by afforestation, we examined N(2) fixation of Elaeagnus angustifolia L. in mixed plantations with non-fixing Populus euphratica Oliv. and Ulmus pumila L. Fixation of N(2) was quantified by the (15)N natural abundance technique based on both foliar and whole-plant sampling during five consecutive growing seasons. Despite elevated root-zone soil salinity (6-10 dS m(-1)) and deficiency in plant-available P (4-15 mg kg(-1)), N(2) fixation (%Ndfa) increased from an initial value of 20% to almost 100% over 5 years. Within each growing season, %Ndfa steadily increased and peaked in the fall. Annual N(2) fixation, determined using foliar delta(15)N, initially averaged 0.02 Mg ha(-1), peaked at 0.5 Mg ha(-1) during the next 2 years and thereafter stabilized at 0.3 Mg ha(-1). Estimates based on whole-plant delta(15)N were <10% lower than those based on foliar delta(15)N. The increase in plant-available soil N was significantly higher in E. angustifolia plots than in P. euphratica and U. pumila plots. Increases in the concentrations of organic C (19%), total N (21%) and plant-available P (74%) in the soil were significant irrespective of tree species. This improvement in soil fertility is further evidence that afforestation with mixed-species plantations can be a sustainable land use option for the degraded irrigated croplands in CA.