As competition among industrial, agricultural, urban and environmental sectors for freshwater intensifies, wastewater is frequently being seen as a valuable resource rather than mere waste. Furthermore, wise reuse of this water alleviates environmental concerns attendant with its discharge to coastal environments and inland waterways. Globally, around 20 million ha of land are irrigated with wastewater, either neat or partially diluted. This figure is likely to increase markedly over the next few decades in response to rising levels of water stress in inhabited catchments -in 1995 around 2.3 billion people lived in river basins considered to be water stressed and this number is expected to increase to 3.5 billion by 2025. Here we review the current status of wastewater irrigation by providing an overview of the extent of the practice in different parts of the world and through an assessment of the current understanding of various issues relating to sustainable and safe management of irrigation with wastewater. A theme that emerges is that wastewater irrigation is not only more common in water stressed regions such as the Near East, but the rationale for the practice also tends to differ between the developing and developed worlds. In developing nations the prime drivers for wastewater irrigation appears to be livelihood dependence and food security, whereas environmental agenda appear to hold greater sway in the developed world. The following were identified as key areas requiring greater understanding for the long-term sustainability of wastewater irrigation: (i) accumulation of bio-available forms of heavy metals in soils, (ii) an understanding of the balance of various factors affecting the environmental fate of organics in wastewater irrigated soils (iii) the influence of reuse schemes on catchment hydrology, including transport of salt loads, (iv) risk models for helminth infections (pertinent to developing nations), (v) microbiological contamination risks for aquifers and surface waters, (vi) transfer efficiencies of chemical contaminants from soil to plant, (vii) effects of chronic exposure of people to chemical contaminants in wastewater, and (iix) strategies for engaging the public in wastewater irrigation schemes.

[1] This paper presents the characterization and validation of retrievals of atmospheric methane (CH 4 ) vertical profiles by the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua platform. AIRS channels near 7.6 mm are used for CH 4 retrieval, and they are most sensitive to the middle to upper troposphere, i.e., about 200-300 hPa in the tropics and 400-500 hPa in the polar region. The atmospheric temperature-humidity profiles, surface skin temperature, and emissivity required to derive CH 4 are obtained from retrievals using separate AIRS channels and the Advanced Microwave Sounding Unit (AMSU). Comparison of AIRS retrieved profiles with some in situ aircraft CH 4 profiles implied that the forward model used in the AIRS retrieval system V4.0 required a 2% increase in methane absorption coefficients for strong absorption channels, and this bias adjustment was implemented in the AIRS retrieval system V5.0. As a new operational product in V5.0, AIRS CH 4 were validated using in situ aircraft observations at 22 sites of the NOAA Earth System Research Laboratory, Global Monitoring Division (NOAA/ESRL/GMD), ranging from the Arctic to the tropical South Pacific Ocean, but their altitudes are usually above 300 hPa. The results show the bias of the retrieved CH 4 profiles for this version is À1.4$0.1% and its RMS difference is about 0.5-1.6%, depending on altitude. These validation comparisons provide critical assessment of the retrieval algorithm and will continue using more in situ observations together with future improvement to the retrieval algorithm. AIRS CH 4 products include not only the CH 4 profile but also the information content. As examples, the products of AIRS CH 4 in August 2004 and the difference of CH 4 in May and September 2004 are shown. From these results a few features are evident: (1) a large AIRS CH 4 plume southwest of the Tibetan plateau that may be associated with deep convection during the Asian summer monsoon; (2) high mixing ratios of AIRS CH 4 in southeastern Asia and in the high northern hemisphere in the summer; and (3) the increase of AIRS CH 4 from May to September in the high northern hemisphere that is likely linked with wetland emission but needs more study. Further analysis of these data and its comparison with model data will be addressed in a separate paper.

MODIS Protoflight Model, launched on-board the NASA EOS Terra spacecraft on December 18, 1999, has been in operation for more than two years, providing the science community calibrated data sets for the global studies of the land, oceans, and atmosphere. MODIS has 36 spectral bands, covering the spectral range from 412 to 14200 nm, and provides spatial resolutions of 0.25 km (2 bands), 0.5 km (5 bands), and 1 km (29 bands) at nadir. The key On-board Calibrators (OBCs) include a Solar Diffuser (SD) and a Solar Diffuser Stability Monitor (SDSM) system for the reflective solar bands calibration, and a V-grooved flat panel Blackbody (BB) for the thermal emissive bands calibration. In this paper, we describe the use of OBCs for the sensor s radiometric calibration and characterization and discuss on-orbit performance. In addition, we provide an assessment on the SD and MODIS optics on-orbit degradation.