Hey Zeinab! You can't. And even if you could, you would most likely fall into some serious errors in your modeling if you're modeling small-scale events.

That is because watersheds often have their response in durations less than 30 days. Most likely your case is of a watershed whose hydrologic cycle components such as precipitation, evaporation, infiltration, and runoff have durations much less than 30 days. If you're familiar with the concept of time of concentration, you most likely will have noticed that it is often less than 30 days.

That is why, for example, most hydrologic studies and hydraulic design guidelines recommend adopting a smaller duration, e.g. 24 hours. Research at least as old as Wiesner (1970) states that certain patterns of precipitation can be identified if you're dealing with rainfalls with more than 24 hours. The U.S. Natural Resources Conservation Service (2019) also recommends developing synthetic rainfall distributions in a 24-hour pattern; its suggested method ensures that the maximum rainfall of any duration less than 24 hours is included in the distribution.

If you're working with inputting time-series data in your HEC-HMS model, you must've noticed that it isn't possible to include a time interval of more than 1 day (as precipitation gages). Similarly, time intervals that control your simulation (as control specifications) can only be up to 1 day long. HEC-HMS itself only features monthly intervals while simulating constant baseflow, evaporation, and average evapotranspiration, according to their technical reference manual (Hydrologic Engineering Center, 2022).

What I suggest you to do is expanding your search and finding a platform that offers more discretized data for your region of interest. One example I can think of on the top of my head is Renewables.ninja, developed by Imperial College London and ETH Zurich (Gelaro et al., 2017). It offers hourly rainfall intensities for anywhere in the world. You could find one that is more suitable to you. Good luck!

References (in Elsevier Harvard style format):

Gelaro, R., McCarty, W., Suárez, M.J., Todling, R., Molod, A., Takacs, L., Randles, C.A., Darmenov, A., Bosilovich, M.G., Reichle, R., Wargan, K., Coy, L., Cullather, R., Draper, C., Akella, S., Buchard, V., Conaty, A., Silva, A.M. da, Gu, W., Kim, G.-K., Koster, R., Lucchesi, R., Merkova, D., Nielsen, J.E., Partyka, G., Pawson, S., Putman, W., Rienecker, M., Schubert, S.D., Sienkiewicz, M., Zhao, B., 2017. The modern-era retrospective analysis for research and applications, version 2 (MERRA-2). Journal of Climate 30, 5419–5454. https://doi.org/10.1175/JCLI-D-16-0758.1

Hydrologic Engineering Center, 2022. HEC-HMS technical reference guide (Computer program documentation No. CPD-74B). U.S. Army Corps of Engineers, Davis.

Natural Resources Conservation Service, 2019. Storm rainfall depth and distribution, in: Hydrology, National Engineering Handbook. U.S. Department of Agriculture, Washington, pp. 630-4.i-630–4.69.

Wiesner, C.J., 1970. Hydrometeorology. Chapman and Hall.