Abstract: Hard water is used for drip irrigation in some areas of China due to precipitation and groundwater resource limitation. But it is proved that drip irrigation emitters can be clogged when using hard water for irrigation. In order to explore the influence of temporary hard water on emitter clogging and clogging mechanism in drip irrigation system, a laboratory experiment was conducted to investigate the variations of the emitter's average relative flow rate and the distribution of clogged emitters along the drip line and the clogging materials in the clogged emitters. The experiment started in April 2014 at the Irrigation Hydraulics Laboratory of Northwest A&F University. Two kinds of emitters (internal cylindrical type and internal inserting type) were used and water hardness was set at 3 levels (0, 250 and 500 mg/L) in this experiment. Corresponding to 3 kinds of irrigation water, 3 independent and identical drip irrigation systems with cyclic water were built. Each system has 4 drip lines (2 drip tubes with internal cylindrical emitter and 2 drip tapes with internal inserting emitter). And there were 18 emitters on each drip line. The total duration of irrigation was 35 d, with a daily application of 4 h (from 8:00 to 12:00) under working pressure of 100 kPa. Flow rates of all the emitters were measured once every 7 days using the weighting method. The clogging in the labyrinth passage was analyzed by the field emission scanning electron microscope (FESEM) after all irrigation events. The results indicated that the water hardness had a very significant influence on the level of emitter clogging. For the 2 kinds of emitters, the average relative flow rates were maintained at more than 95% of the original flow rate during whole experiment with 0 hardness water, and none of emitters was clogged; the average relative flow rates were 51.1% and 59.4% respectively for internal cylindrical emitter and internal inserting emitter with 250 mg/L hardness water, and the percentages of clogging emitter were 50% and 41.7% respectively; with 500 mg/L hardness water, the average relative flow rates were 15.4% and 19.1% and the percentages of clogging emitter were 100% and 97.2% respectively. Therefore, hard water can cause emitter clogging in drip irrigation, which manifests as the decrease of emitter flow rate. And the harder the water, the less the flow rate. But in the experiment, the average relative flow rates and the clogging percentages were nearly the same for the 2 kinds of emitters with the same water hardness. The clogged emitters were distributed along the drip line uniformly including upper 1/3, middle 1/3 and lower 1/3 segments. There was no obvious clogging distribution pattern for each part of the drip line. FESEM analysis showed that there was an adhesion layer formed by white sediment attaching on the surface of emitter flow path. The sediment was formed by crystal particles. And the crystal particles were connected very closely. Energy spectrum analysis showed that the white sediment was composed by elements O (oxygen), Ca (calcium), C (carbon) and very small amounts of Fe (iron). It could be concluded that white sediment was a mixture of CaCO3 and a very small amount of other chemical precipitations. So, the main reason for emitter clogging is chemical clogging caused by CaCO3 precipitation. To maintain a high system performance, the water temporary hardness should be less than 250 mg/L at least. The results will provide a reasonable proposal and a theoretical reference to understand emitter clogging mechanism for hard water drip irrigation.