  In this paper we describe the photometric calibration of data taken with the
near-infrared Wide Field Camera (WFCAM) on the United Kingdom Infrared
Telescope (UKIRT). The broadband ZYJHK data are directly calibrated from 2MASS
point sources which are abundant in every WFCAM pointing. We perform an
analysis of spatial systematics in the photometric calibration, both inter- and
intra-detector and show that these are present at up to the 5 per cent level in
WFCAM. Although the causes of these systematics are not yet fully understood, a
method for their removal is developed and tested. Following application of the
correction procedure the photometric calibration of WFCAM is found to be
accurate to approximately 1.5 per cent for the JHK bands and 2 per cent for the
ZY bands, meeting the survey requirements. We investigate the transformations
between the 2MASS and WFCAM systems and find that the Z and Y calibration is
sensitive to the effects of interstellar reddening for large values of E(B-V)',
but that the JHK filters remain largely unaffected. We measure a small
correction to the WFCAM Y-band photometry required to place WFCAM on a Vega
system, and investigate WFCAM measurements of published standard stars from the
list of UKIRT faint standards. Finally we present empirically determined
throughput measurements for WFCAM.
