  Alternative to weak lensing measurements through cosmic shear, we present a
weak lensing convergence $\hat{\kappa}$ map reconstructed through cosmic
magnification effect in DECaLS galaxies of the DESI imaging surveys DR9. This
is achieved by linearly weighing $12$ maps of galaxy number overdensity in
different magnitude bins of $grz$ photometry bands. The weight is designed to
eliminate the mean galaxy deterministic bias, minimize galaxy shot noise while
maintaining the lensing convergence signal. We also perform corrections of
imaging systematics in the galaxy number overdensity. The $\hat{\kappa}$ map
has $8365$ deg$^2$ sky coverage. Given the low number density of DECaLS
galaxies, the $\hat{\kappa}$ map is overwhelmed by shot noise and the map
quality is difficult to evaluate using the lensing auto-correlation.
Alternatively, we measure its cross-correlation with the cosmic shear catalogs
of DECaLS galaxies of DESI imaging surveys DR8, which has $8365$ deg$^2$
overlap in sky coverage with the $\hat{\kappa}$ map. We detect a
convergence-shear cross-correlation signal with $S/N\simeq 10$. The analysis
also shows that the galaxy intrinsic clustering is suppressed by a factor
$\mathcal{O}(10^2)$ and the residual galaxy clustering contamination in the
$\hat{\kappa}$ map is consistent with zero. Various tests with different galaxy
and shear samples, and the Akaike information criterion analysis all support
the lensing detection. So is the imaging systematics corrections, which enhance
the lensing signal detection by $\sim 30\%$. We discuss various issues for
further improvement of the measurements.
