Usage information for xml.cgi:

Example:
  xml.cgi?id=MMRW1&pe=QR

Physical ParameterUsageDescription
idid=MMRW1NWS 5 character station identifiers (HB5 codes). The last two characters are state codes (eg W1=WA).
pepe=QRPhysical Element is one of HG, HG, HT, HP, HL, HM, QR, QI, QA.
  Where HG river gage height (ft).
  HF, HT, HP, HL = resevoir or lake pool heights (ft) (forebay, tailrace, pool, lake).
  HM = used for gages in/near the ocean (eg the Columbia near Astoria).
  QR, QI, QA = discharge (cfs) (streamflow, inflow, and adjusted to natural).
numdaysnumdays=3The number of lookback days for observation data. Maximum is 10.
dtypedtype=bDisplays observations(o), forecasts(f), or both(b). Default is observations(o).

Reference:
  SHEF Information
  Download SHEF Manual (PDF)