Q2. What are the future works mentioned in the paper "Ob’ river discharge from topex/poseidon satellite altimetry (1992–2002)" ?
T/P-derived discharge estimates and other hydrological parameters, such as dates of the beginning and the end of spring flood, in combination with other hydrometeorological data ( air temperatures, precipitation, snow cover extent and volume, etc. ) will provide valuable information for studies of water budget and its variability for the whole Ob ’ watershed or selected parts and is the aim of future research. Hydrologic sensitivity is one of the main control variables that determines the future response of the Arctic regions to large-scale climate changes.
Q3. What corrections are applied to the T/P altimeter?
The corrections applied include ionospheric, dry tropospheric, solid Earth tide corrections and correction for the satellite’s centre of gravity.
Q4. What causes the water level to increase in the autumn?
In autumn, the formation of young ice often causes temporal water level increase related to the constriction of the river channel cross section.
Q5. How wide are the rivers monitored from space?
With a new generation of radar altimeters dedicated to continental hydrology, rivers with width on the order of 100 m could be monitored from space.
Q6. What is the stage–discharge rating for a given point?
the stage–discharge rating for a given point consists of the whole family of curves corresponding to different periods when the flow is assumed to be steady.
Q7. What is the contribution of spatial observations to continental hydrology?
The contribution of spatial observations to continental hydrology is likely to develop increasingly in the near future: besides the new radar altimeters on board Jason and ENVISAT, the gravimetric mission GRACE will soon provide the water mass spatio-temporal variations at global and regional scale of 200 km, offering another validation source for the new global hydrologic models that are currently developed.
Q8. What is the time to monitor rivers discharge?
The TOPEX/Poseidon radar altimeter is on a 10-day repeat orbit, well suited to monitor rivers discharge variations, while the 35-day repeat orbit of the ERS altimeters is too coarse especially for Arctic rivers who are subject to intense increase in discharge over 1- or 2-month periods in the spring when snow melts.
Q9. How many days did the authors average the water level measurements?
the authors averaged the selected 1/10 s level measurements and constructed the T/P water level time series over the orbital cycle (10-day interval).
Q10. What is the effect of the presence of ice and snow on the altimeter measurements?
For the relatively flat lower Ob’ region near Salekhard, the presence of ice and snow (on land and on river ice) perturbs the altimeter measurements, which are strongly attenuated by their presence (Kouraev et al., 2003a; Papa et al., 2002).
Q11. What is the way to measure the reflected signal on water?
In other cases, the instrument could remain locked on water while the satellite is well ahead of the water body, since the reflected signal on water has more power than the reflected signal on land.
Q12. What is the TMR footprint of the satellite?
When the satellite flies over rivers, the TMR footprint almost always includes surrounding lands, which contaminates the measurements and makes atmospheric water vapor measurement unreliable.
Q13. What are the main factors controlling the rating curve for rivers with vast flood plains?
For rivers with vast flood plains like the Ob’, the rating curve consists of several branches corresponding to the different hydraulic conditions (or hydrological phases) (Bykov & Vasiliev, 1973).
Q14. How many months of data are missing from the ground track?
In this case, when the T/P data for ground track 187 are missing for more than five consecutive cycles (1.5 month), using the monthly discharge data calculated from T/P observations for ground track 112 increases the accuracy.
Q15. What is the typical asymmetry in the Ob’ hydrographic network?
The Ob’ hydrographical network is characterised by a sharp asymmetry—most of the watershed area (67% of the total area) is located on the left-bank.
Q16. What is the reason for the large errors of annual discharge estimations?
The large errors of annual flow estimations noted in 1995 and 1999 years are caused by the interpolation of discharge estimations when T/P data were not available.
Q17. What is the standard deviation of the 1/10 s level measurements available for each cycle at ground?
The standard deviation of the 1/10 s level measurements available for each cycle at ground track 187 changes from 40 cm during spring flood to 23 cm during water level decrease in late summer early fall.
Q18. What is the common feature of the Ob’ hydrographic network?
Another typical feature is the presence of areas of inner discharge (not providing inflow to the Ob’ river system), which cover 15% of the watershed area.
Q19. How many errors are there between the two estimates?
A comparison of the annual discharges from the T/P water levels with the in situ data (Table 1) shows that the errors between the two estimates are about 400 m3/s or 3% (median values) of mean annual river discharge.