Grass has suffered considerably with the recent dry weather
Where there has been no irrigation, grass cover has suffered considerably with the recent dry weather. However, grass will recover once moisture builds up in the soil and the moisture deficit is overcome. There will be the accumulation of dead surface growth and ideally, this should be scarified out at the end of the summer.
Even without doing so, the green grass cover will be restored and the dead tissue will break down to an extent and add to the organic matter content in the surface layers. This could restrict surface drainage to a degree but should not impair the performance of slit drains significantly.