{"id":4640,"date":"2025-05-10T13:56:19","date_gmt":"2025-05-10T13:56:19","guid":{"rendered":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/?post_type=ht_kb&p=4640"},"modified":"2025-09-05T13:04:51","modified_gmt":"2025-09-05T13:04:51","slug":"malcom-small-watershed-hydrographs","status":"publish","type":"ht_kb","link":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/knowledge-base\/malcom-small-watershed-hydrographs\/","title":{"rendered":"Malcom Small Watershed Hydrographs"},"content":{"rendered":"\n
The Malcom method hydrograph, also known as the Small Watershed Hydrograph Method, originated from a watershed research study led by H. Rooney Malcom in 1980 in North Carolina.<\/p>\n\n\n\n
The Malcom hydrograph is constructed in a way that closely resembles the shape of the NRCS Unit hydrograph. It employes two distinct equations to compute flows, Qi<\/sub>, at a given time, ti<\/sub>.<\/p>\n\n\n\n
Malcom Method Equations<\/h2>\n\n\n\n
The Malcom Hydrograph combines NRCS with the Rational method.<\/figcaption><\/figure>\n\n\n\n
<\/figure>\n\n\n\n
<\/figure>\n\n\n\n
<\/figure>\n\n\n\n
Where:<\/p>\n\n\n\n
Tp<\/sub> = Time to Peak V = Volume (cuft) = Excess precipitation, Q, of the currently selected NRCS design storm (in) x Drainage Area (ac) x 43,560\/12 Qp<\/sub> = Peak discharge as calculated by the Rational method (Qp<\/sub> = CiA) Ti<\/sub> = Time in seconds Qi<\/sub> = Discharge in cfs at Ti<\/sub><\/p>\n\n\n\n
Note that the total precipitation, P, is pulled from the currently selected design storm. The excess precipitation, Q, is calculated from the following curve number equation:<\/p>\n\n\n\n
<\/figure>\n\n\n\n
Where:
Q = Excess volume of precipitation (in) P = Total precipitation (in) for the selected design storm S = Potential maximum retention = (1000 \/ CN) \u2013 10 CN = Curve Number (Hydrology Studio\u2019s published tables of Curve Numbers assumes an antecedent moisture condition of II)<\/p>\n\n\n\n
It is essential to recognize that this method necessitates both a Curve Number (CN) and a Runoff Coefficient (C). These values must accurately reflect their associated land use. Below is a straightforward approach to derive an equivalent CN from a Rational Method C. It is logical to conclude that the Rational Method C indicates a percentage of imperviousness; for instance, a C value of 0.60 signifies 60% impervious surface. An equivalent CN can be calculated as:<\/p>\n\n\n\n
<\/figure>\n\n\n\n
Where:<\/p>\n\n\n\n
IMP = % impervious X is taken from the table below and is based on the Soil Type.<\/p>\n\n\n\n
\n\n
\n\t
Soil Type<\/th>
A<\/th>
B<\/th>
C<\/th>
D<\/th>\n<\/tr>\n<\/thead>\n
\n
\n\t
X =<\/td>
39<\/td>
61<\/td>
74<\/td>
80<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n\n\n
For a C = 0.60 and a Soil Type C, an equivalent CN is 98(.60) + 74(1-0.60) = 88.40.<\/p>\n\n\n\nA Malcom method hydrograph combines the advantages of Rational and NRCS<\/figcaption><\/figure>\n\n\n\n
The Malcom method combines the advantages of both approaches, offering the straightforward Peak Q from the Rational method alongside the realistic volume estimates provided by the NRCS method.<\/p>\n","protected":false},"excerpt":{"rendered":"
The Malcom method hydrograph, also known as the Small Watershed Hydrograph Method<\/p>\n","protected":false},"author":1,"comment_status":"closed","ping_status":"closed","template":"","format":"standard","meta":{"footnotes":""},"ht-kb-category":[37],"ht-kb-tag":[75,62],"class_list":["post-4640","ht_kb","type-ht_kb","status-publish","format-standard","hentry","ht_kb_category-runoff-hydrographs-and-processing","ht_kb_tag-malcolm","ht_kb_tag-malcom-method"],"_links":{"self":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/4640","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb"}],"about":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/types\/ht_kb"}],"author":[{"embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/comments?post=4640"}],"version-history":[{"count":4,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/4640\/revisions"}],"predecessor-version":[{"id":4644,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/4640\/revisions\/4644"}],"wp:attachment":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/media?parent=4640"}],"wp:term":[{"taxonomy":"ht_kb_category","embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb-category?post=4640"},{"taxonomy":"ht_kb_tag","embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb-tag?post=4640"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
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